X11: Update modifiers with key events

[murarth]

• 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

[murarth]

Solves #1124 for X11, but not Wayland.

Note that I've made no changelog entry yet. I'm hoping maybe I can add a fix for Wayland to this PR before it's merged.

[elinorbgr]

FTR, I'm not sure #1124 needs solving: #1124 (comment), so I'd like that merging this PR waits that we reach a consensus in #1124 first.

[murarth]

Closing in favor of a different solution.