Adjust the `ModifiersChanged` event handling

[kchibisov]

The spec got one more "clarification" lately wrt the keyboard modifiers. Given that our ModifiersChanged event is tied to Focus state, as in on leave we don't have focus and on regaining focus we have modifiers back on.

For winit's event API it simply means that we should send the ModifiersChanged event along side the PointerEnter and PointerLeave events. I'm not sure whether it's a breaking change, but it's something we should consider once we land the #2662 (Keyboard input v2).

The backends should start sending this event when pointer enters the surface of the window, as well as sending it again on keyboard enter/leave.

The good thing is that we still assume the empty state when losing, but then compositor should send the modifiers event again to tell us about the pointer focus. Usually compositors will send
such events related to pointer and keyboard focuses, so we should have internal seat focus inside
the Wayland backend.

For more see:

https://gitlab.freedesktop.org/wayland/wayland/-/merge_requests/259

[kchibisov]

Hm, hard to find compositor that actually does so to test. Will move to 0.31 for now.