Bug fix: UTF-8 support & misc

- Add UTF-8 support to tooltip. I didn't correct the potential charset
  issues in wm_get_window_title(), as I'm considering a rewrite of it.

- Change multiple things in Makefile. Switch to C99. Add git-hash-based
  version number.

- Drop environment.h and use "long" instead in wm_get_stack().

- Fix compilation warnings.

- Known issue: Probably due to a bug in nvidia-drivers's X Render
  transformation implementation, window content is incorrectly
  positioned here.