Add Plymouth: WIP
Plymouth: works quite well with the X11 renderer and from a VT

I haven't figured out how to get it into the initrd correctly yet.
Merge pull request #460 from cillianderoiste/plymouth

It works enough to display bootsplash animations in an xorg session and a VT.

I haven't figured out how to run it successfully from the initrd yet and I'm also not happy with the postInstall mess, but I'd rather merge it now than let it get lost. It seems like it should be possible for a user to activate it by using boot.initrd.extraUtilsCommands and boot.initrd.postMountCommands