Add safe constructors for display and window handles

[notgull]

There are some DisplayHandle and WindowHandle variants that are completely safe to construct. For instance:

• Most DisplayHandles are just indicators of the currently running display system and don't involve any borrowed state.
• An [Xlib/Xcb]DisplayHandle with a display of None can be constructed safely.
• Win32WindowHandle, [Xlib/Xcb]WindowHandle and WASM web handles involve window IDs with no borrowed state.

It should be possible to construct these safely, with constructors on the safe types.

[kchibisov]

The reason everything in unsafe is forward compat, I guess? Because unsafe -> safe is a breaking change, but not otherwise.

[kchibisov]

Also, isn't Default basically it?

[notgull]

I was thinking more along these lines:

impl DisplayHandle<'static> {
    pub fn windows() -> Self {
        unsafe { Self::borrow_raw(WindowsDisplayHandle::new().into()) }
    }

    pub fn xlib_no_display() -> Self {
        unsafe { Self::borrow_raw(XlibDisplayHandle::new(None).into()) }
    }
}

Basically, to allow for more ways of creating DisplayHandle and WindowHandle without needing to go through borrow_raw.