[branch-merge] Add x11 renderer plugin

This merges the "x11-renderer" branch to master.

In order to make debugging splash plugins easier,
and in order to make debugging multi-head renderering
possible on single head displays, Charlie wrote an
X11 rendering plugin.

This plugin displays plymouth in a running X session,
instead of on the console.

It currently only supports graphical plugins.  At some
point it may grow support for text plugins, too, using
vte.  That will take some reworking of the plymouth
daemon core.

This could some day serve as a basis for providing a
graphical theme chooser application (like
gnome-screensaver-preferences), although it's not clear
yet that's a good idea.

Right now, it's a great debugging and development tool,
though.