Add command for setting mouse passthrough

[jquesada2016]

This PR addresses #2283 by exposing winit's ability to set mouse hit testing.

I extended the window::Action enum to include two new entries:

• EnableMousePassthrough: enables the window passing through mouse events to whatever's underneath
• DisableMousePassthrough: disables mouse passthrough, essentially allowing the window to capture mouse events

I also added two respective functions for creating these commands, enable_mouse_passthrough, and disable_mouse_passthrough.

[dtzxporter]

May be a little verbose to have two actions when you could just make it iced::window::set_cursor_hittest(id, enabled) instead?

[jquesada2016]

May be a little verbose to have two actions when you could just make it iced::window::set_cursor_hittest(id, enabled) instead?

@dtzxporter I do personally agree. I ended up splitting it into both because I tried sticking to the rest of the window::Action variants, and it felt more consistant this way. Also, using Set in the name implies to me there should be a mechanism for fetching the hit test state, which winit does not have.