ComptonConfiguration: switched to libconfig as a configuration backend.

To make this possible, a dirty vapi binding has been written (it ships in vapis/).
It contains only the methods that are useful for us now, and it's pretty bad written.
libconfig.vapi: set_* methods return only a status boolean.
compton: added write support.

Whenever a setting is changed in dconf, it will immediately syncronized in the
compton configuration file.