Add optional dbus helpers

Add optional dbus helpers

This adds optional libdbus integration for weston. If libdbus is available
and not disabled via --disable-dbus during weston build, we now provide
basic DBusConnection main-loop integration for weston.

The dbus.c file provides a new helper to integrate any DBusConnection
object into a wl_event_loop object. This avoids any glib/qt/..
dependencies but instead only uses the low-level libdbus library.

Note that we do not provide dummy fallbacks for dbus helpers in case
dbus-support is disabled. The reason for that is that you need dbus/dbus.h
for nearly any operation you want to do via dbus. Therefore, only the most
basic helpers which can be used independently provide a "static inline"
dummy fallback to avoid #ifdef all over the code.