Improvement #74: Use libevent for main loop

- Use libevent for main loop. I will explain the reasons in #56 later.
  The preferred libevent version is 2.x, yet 1.4.x should work as well.

- As a result, compton now should build fine on *BSD. Thanks to
  DachiChang for the FreeBSD build issue report.

- Another consequence is we now use microsecond-level timing all the
  way. Nanosecond-level code will be dropped soon. Start using long
  instead of unsigned long to represent time in milliseconds, as both
  can't hold the full epoch time in ms, anyway, and a signed type
  requires less care in subtraction. Wrap the epoch time in ms to 15
  days.

- Fix broken NO_VSYNC_DRM and NO_VSYNC_OPENGL compile-time options.

- Use git revision number for versioning in Makefile, and other small
  improvements.

- Reorganize struct _win. Drop unused w->damaged_sequence. w->damaged is
  turned to bool.

- Add type and format to winprop_t, as preparation for the new condition
  format.

- Add w->shadow_force and w->focus_force, to prepare for D-Bus support.

- Rename wid_get_prop() to wid_get_prop_adv() with more options. Add
  wrapper function wid_get_prop().

- Add some extra helper functions, for D-Bus support later.

- Make some functions return a bool value to indicate if it's
  successful.

- Modify add_win(), use a static const structure to initialize the new
  struct _win.

- Add some helper macros, like printf_err(f)(q). Make some errors fatal.

- Rename some types, constants, and functions. Code clean-up.

- Check for time disorder in paint_preprocess() when calculating fading
  steps.

- Rename evpoll() to swopti_handle_timeout(), and partially rewrite it.

- Make -h / --help legal.

- Known issue: compton segfaults on FreeBSD with nvidia-drivers, unless
  NO_VSYNC_OPENGL is used. Will look into it later. Thamls to DachiChang
  for reporting.

README.md

@@ -56,6 +56,7 @@ __R__ for runtime
 * libdrm (B) (Will probably be made optional soon)
 * libGL (B,R) (Will probably be made optional soon)
 * asciidoc (B) (if you wish to run `make docs`)
+* libevent (B,R)
 
 ### How to build