[compiz] configuration support in dbus plugin

[David Reveman]

I've added configuration support to the dbus plugin. I haven't done much
testing so let me know if you find any issues with it. You can find some
usage examples in the dbus plugin code.

This is what KDE needs for compiz configuration support but you can of
course use it for other configuration systems too. E.g. writing a simple
daemon that monitors a text file and use this dbus interface should be
trivial if someone wants to do that.

There's still things missing in the dbus plugin. Like introspection,
ability to query available options and their restrictions.. I'll add
these things once someone needs them, or even better, I'll accept
patches from someone who implements them.

Simple example for how it can be used with dbus-send for command line
configuration of compiz:

dbus-send --type=method_call --dest=org.freedesktop.compiz \
/org/freedesktop/compiz/core/allscreens/command0 \
org.freedesktop.compiz.set string:'gnome-terminal'

-David