Modifier key shortcuts not responsive

[csiew]

Key	Description
Problem	Keybindings with modifier key are non-responsive.
Report message	"I can move the mouse, but any of the keybindings with the keyboard doesn't work. Alt-Space or Alt-Esc or Alt-X ...nothing"
Report date	18 May 2021

[csiew]

Ticket: https://trello.com/c/Wz9EoLK8

First step towards resolving this by refactoring the whole keyboard event observer.

Known issues

• Keyboard events are currently completely consumed by BiscuitWM and can't pass them to applications (very bad).
• When using Xephyr, the Shift key is not removed from the key_down array. This means that anyone using BiscuitWM on native hardware will likely have to press Shift when using any of the BiscuitWM keyboard shortcuts.