xwayland: Refactor xwayland

Over time, Xwayland main source file xwayland.c has grown in size which makes it look cluttered and harder to read.

Refactor the source code to have separate header files per source file.

I know this is a pretty big set of changes, but that should not introduce any functionality change and I reckon this is necessary to make the Xwayland source code a bit prettier, so the sooner the better...