Switch from Xlib to xcb

[DemiMarie]

The problem you're addressing (if any)
Xlib is an old library that is bloated, inefficient, and makes error handling difficult. We should switch to xcb instead. Xlib Display pointers are also valid as an xcb_connection_t and can be cast to the latter.

Describe the solution you'd like

Switch (incrementally) to xcb.

Where is the value to a user, and who might that user be?

All users will benefit from improved graphics performance.

Describe alternatives you've considered

Do nothing until we switch to Wayland

Additional context

This might be a terrible waste of resources. I will let @marmarek decide 😄.

Relevant documentation you've consulted

Related, non-duplicate issues

#3366

[marmarek]

It isn't really worth the effort, lets use that time for Wayland (#3366) instead.