simplify client_for_each_surface
All the XDG surface iterator does is iterate the main wlr_surface, then
iterate the popups. If we inline that function, we can merge part of it
with the X11 case.
consolidate some of the ugliness into a separate file
Similar to Linux kernel approach, encapsulate some of the uglier
conditional compilation into inline functions in header files.
The goal is to make dwl.c more attractive to people who embrace the
suckless philosophy - simple, short, hackable, and easy to understand.
We want dwm users to feel comfortable here, not scare them off. Plus,
if we do this right, the main dwl.c code should require only minimal
changes once XWayland is no longer a necessary evil.
According to `cloc`, this also brings dwl.c down below 2000 lines of
non-blank, non-comment code.