Removing fallback support code

[Bastien Nocera <hadess hadess net>]

Heya,

I've gone through gnome-settings-daemon, gnome-control-center and
gnome-bluetooth, and removed quite a few of the fallback supporting
features.

I've "tagged" the commits with "#fallback" so that you can find them
easily.

If you used the Bluetooth applet (LXDE, XFCE), you can base your code on
the bluetooth-applet.h helper.

If you used the volume control applet (Unity), you can revive the code
in a separate module and use the libgnome-volume-control git sub-module.

If your environment doesn't have automounting, you can revive the code
from gnome-settings-daemon in a separate module.

IBus support is always turned on (though you can still build without
ibus support, or remove the ibus binary at run-time).

There's going to be more changes in the future including:
- removing gnome-screenshot as a dependency for taking screenshots
- move background rendering to the shell
- move media-keys handling to the shell (or at least fix shortcuts in
the overview)

Cheers