no need to cast NULL to void *

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.

Stivvo deserves a shout-out too!

fix build with -DXWAYLAND

missed a few of these

don't call applyexclusive just to return

let's not alienate the dwm folks

Fix style.

Merge updates from guidocella

Thanks so much for helping to keep the project running while life was
crazy!

Merge pull request #32 from alex-courtis/24-xwayland-cursor

#24 ensure that xwayland cursor defaults to left_ptr

simplify ROUND macro

Merge pull request #54 from Bonicgamer/swayscalebox

Sway scale_box (rounding)

Merge pull request #53 from Stivvo/fullscreen

Fullscreen support

fix crash on non-libinput pointers (e.g. X11 backend)

Same target as dwm sounds like a good goal

Merge pull request #49 from guidocella/layer-shell

Layer shell

use the install command to install

Merge pull request #65 from Keating950/master

Add install target to Makefile

Merge pull request #56 from oyren/readme_log

remove log flags from readme

Merge pull request #62 from Java-boi/master

Added basic tap-to-click for touchpad users

Merge pull request #66 from richardipsum/master

fix undeclared WLR_KEY_PRESSED