Add GDBusDAaemon, an implementation of a message bus

This is mostly complete, sans support for activation. However, its
not as picky as the libdbus implementation in terms like validation
and limits checking, nor is it as tested.

Its can be useful to test gdbus if dbus-daemon is not availible, but
its main reason for existance is to implement a default session bus
on win32 so that e.g. GApplication is guaranteed to work.