Cleaning up PKGBUILD

- All changes as suggested by bidulock (thanks!).
- Moving xorg-xmessage to opt_depends() because the buttons script
  checks for the existence of the command anyway.
- Adding xorg-xsetroot and xorg-xwininfo to depends().
- Removing python2 and xorg-utils from depends(). The former wasn't used
  at all, the latter was only used for xorg-xsetroot.