X11: Fix modifiers being reported after release

[murarth]

• Moves ModifiersChanged variant from WindowEvent to DeviceEvent

Fixes #1259

• Tested on all platforms changed
• Compilation warnings were addressed
• cargo fmt has been run on this branch
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

[chrisduerr]

I can't reproduce the bug on my machine, but I can verify that the modifiers now behave the same way on X11 as they do on macOS. So when pressing shift, the modifiers state has shift set both for the shift key press itself and for all following events.

So everything looking good with regards to functionality from a consumer perspective. Thanks for the quick work @murarth, nicely done.

[goddessfreya]

Prior to this PR I was getting this shit: https://gist.github.com/goddessfreya/0192aa8d9920dba7c933c98190c42d90

See those pairs of ModifiersChanged? I'd get them both when releasing Mod-; back onto winit's window. The first is obviously wrong, as mod was held and never released the whole time. Unfortunately, we also didn't get the matching Released for the:

WindowEvent { window_id: WindowId(X(WindowId(46137345))), event: KeyboardInput { device_id: DeviceId(X(DeviceId(3))), input: KeyboardInput { scancode: 125, state: Pressed, virtual_keycode: None, modifiers: ModifiersState { shift: false, ctrl: false, alt: false, logo: false } } } }

While this PR fixes the first issue, it doesn't fix the second, but I guess that is out of scope. Anyways, good job!