Wayland support

[Caellian]

This is a feature request for Wayland support.

In total, I feel there's very little that needs to be added/changed to support Wayland on Linux platform.

Needed changes:

• CMake needs additional option to toggle Wayland support which would then only require XKB from X11 as it's still used by Wayland applications.
• I think it would be more convenient to move src/linux into src/linux/x11 and duplicate into src/linux/wayland and then make CMake switch between different implementations based on configuration flag.
• Instead of querying events directly, provide a pointer to wl_pointer_listener so applications can attach it on seat_capability events.
• Keyboard events are handled very similarly to X11 (using XKB), only the listener registration is handled like mouse events in previous point.

Reference:

• Conky (mouse events)