GDBus is more and better maintained than libdbus these days. In the
future, a potential Wayland-compatible capture plugin will need to
interact with D-Bus in a way that's way too complicated for libdbus,
and it won't be nice to have both libraries talking to the D-Bus
socket.
Replace the libdbus usage by GDBus. As it turns out, it results in less
code.
ec1b07cc85