Bug fix #84: Root window not repainted sometimes on wallpaper change

- Fix a bug that root window is not repainted on wallpaper change unless
  an Expose X event is received. Seemingly, if there's no mapped window
  on a screen, X will not send an Expose event when the wallpaper
  changes. Thanks to baskerville for reporting.

- Fix a X Pixmap leak when there's no existing wallpaper pixmap found.

- Fix a bug in mstrncpy() that null character is not added to the end of
  the copied string.

- Make VSYNC_STRS public, for use in src/dbus.c. Adjust the type of
  WINTYPES array. Add NUM_VSYNC.

- Add more targets for various D-Bus methods. Add "bad_target" D-Bus
  error. Improve error handling. Add more helper functions to append
  arguments to a D-Bus message. Add Introspect method to D-Bus
  introspection reply.

- Add public declarations of things in the new condition format code to
  common.h. Move definitions of some inline functions from compton.h to
  common.h. Make some functions public. Move wid_get_prop_adv() to
  compton.c. The primary code files of the new format src/c2.{c,h} will
  be published when ready.

- Add support for dumping version string in Makefile (make version), to
  make snapshot generation easier.

- Add repeated inclusion protection to common.h.

- Update documentation.

- Use gsed instead of sed in dbus-examples/cdbus-driver.sh if possible,
  as some BSD systems does not come with GNU sed by default. Thanks to
  DaChiChang for reporting.

- Code clean-up. Small type changes in register_cm() to silence
  warnings. Quit on failure in parse_vsync(). Apply stricter checks in
  force_repaint().