break screen shortcuts: implement on wayland

[deltragon]

Description

Fixes #480.

I've also fixed a bug where on X11, the shortcuts were not disabled in some situations (eg. by the limitconsecutiveskipping plugin). See the second commit.

[deltragon]

How to test this:
Start a break. Wait 2 seconds after the break screen appears. (This delay is configurable by the "Keyboard shortcuts disabled period" setting, and defaults to 2s.)
When skipping is enabled, press the "Escape" key to skip the break. When disabled, "Escape" should do nothing.
When postponing is enabled, press the "Space" key to postpone the break. When disabled, "Skip" should do nothing.