Added the ability to get or set the last change tick of a system. (be…

…vyengine#5838)

# Objective
I'm build a UI system for bevy. In this UI system there is a concept of a system per UI entity. I had an issue where change detection wasn't working how I would expect and it's because when a function system is ran the `last_change_tick` is updated with the latest tick(from world). In my particular case I want to "wait" to update the `last_change_tick` until after my system runs for each entity.

## Solution
Initially I thought bypassing the change detection all together would be a good fix, but on talking to some users in discord a simpler fix is to just expose `last_change_tick` to the end users. This is achieved by adding the following to the `System` trait:
```rust
    /// Allows users to get the system's last change tick.
    fn get_last_change_tick(&self) -> u32;
    /// Allows users to set the system's last change tick.
    fn set_last_change_tick(&mut self, last_change_tick: u32);
```

This causes a bit of weirdness with two implementors of `System`. `FixedTimestep` and `ChainSystem` both implement system and thus it's required that some sort of implementation be given for the new functions. I solved this by outputting a warning and not doing anything for these systems. 

I think it's important to understand why I can't add the new functions only to the function system and not to the `System` trait. In my code I store the systems generically as `Box<dyn System<...>>`. I do this because I have differing parameters that are being passed in depending on the UI widget's system.  As far as I can tell there isn't a way to take a system trait and cast it into a specific type without knowing what those parameters are.

In my own code this ends up looking something like:
```rust
// Runs per entity.
let old_tick = widget_system.get_last_change_tick();
should_update_children = widget_system.run((widget_tree.clone(), entity.0), world);
widget_system.set_last_change_tick(old_tick);


// later on after all the entities have been processed:
for system in context.systems.values_mut() {
    system.set_last_change_tick(world.read_change_tick());
}
```

## Changelog

- Added `get_last_change_tick` and `set_last_change_tick` to `System`'s.

crates/bevy_ecs/src/system/function_system.rs

@@ -406,6 +406,14 @@ where
         out
     }
 
+    fn get_last_change_tick(&self) -> u32 {
+        self.system_meta.last_change_tick
+    }
+
+    fn set_last_change_tick(&mut self, last_change_tick: u32) {
+        self.system_meta.last_change_tick = last_change_tick;
+    }
+
     #[inline]
     fn apply_buffers(&mut self, world: &mut World) {
         let param_state = self.param_state.as_mut().expect(Self::PARAM_MESSAGE);

crates/bevy_ecs/src/system/system.rs

@@ -31,6 +31,7 @@ pub trait System: Send + Sync + 'static {
     fn archetype_component_access(&self) -> &Access<ArchetypeComponentId>;
     /// Returns true if the system is [`Send`].
     fn is_send(&self) -> bool;
+
     /// Runs the system with the given input in the world. Unlike [`System::run`], this function
     /// takes a shared reference to [`World`] and may therefore break Rust's aliasing rules, making
     /// it unsafe to call.
@@ -59,6 +60,14 @@ pub trait System: Send + Sync + 'static {
     fn default_labels(&self) -> Vec<SystemLabelId> {
         Vec::new()
     }
+    /// Gets the system's last change tick
+    fn get_last_change_tick(&self) -> u32;
+    /// Sets the system's last change tick
+    /// # Warning
+    /// This is a complex and error-prone operation, that can have unexpected consequences on any system relying on this code.
+    /// However, it can be an essential escape hatch when, for example,
+    /// you are trying to synchronize representations using change detection and need to avoid infinite recursion.
+    fn set_last_change_tick(&mut self, last_change_tick: u32);
 }
 
 /// A convenience type alias for a boxed [`System`] trait object.

crates/bevy_ecs/src/system/system_chaining.rs

@@ -106,6 +106,15 @@ impl<SystemA: System, SystemB: System<In = SystemA::Out>> System for ChainSystem
         self.system_a.check_change_tick(change_tick);
         self.system_b.check_change_tick(change_tick);
     }
+
+    fn get_last_change_tick(&self) -> u32 {
+        self.system_a.get_last_change_tick()
+    }
+
+    fn set_last_change_tick(&mut self, last_change_tick: u32) {
+        self.system_a.set_last_change_tick(last_change_tick);
+        self.system_b.set_last_change_tick(last_change_tick);
+    }
 }
 
 /// An extension trait providing the [`IntoChainSystem::chain`] method for convenient [`System`]

crates/bevy_time/src/fixed_timestep.rs

@@ -224,6 +224,14 @@ impl System for FixedTimestep {
     fn check_change_tick(&mut self, change_tick: u32) {
         self.internal_system.check_change_tick(change_tick);
     }
+
+    fn get_last_change_tick(&self) -> u32 {
+        self.internal_system.get_last_change_tick()
+    }
+
+    fn set_last_change_tick(&mut self, last_change_tick: u32) {
+        self.internal_system.set_last_change_tick(last_change_tick);
+    }
 }
 
 #[cfg(test)]