dbus: refactor WpDBus into a plugin called dbus-connection

Now that we have proper module load order, we can have this shared
dbus connection in a module instead of the library. The module has
to be loaded before any other modules that need it, obviously.

lib/wp/wp.h

@@ -13,7 +13,6 @@
 #include "component-loader.h"
 #include "conf.h"
 #include "core.h"
-#include "dbus.h"
 #include "device.h"
 #include "error.h"
 #include "event-dispatcher.h"