Add cloud menu background by Krisi.

Also add a menuheader, menusplash, and allow HD menu footers.
And finally don't git-ignore textures/base/.
Screenshot: 
![Minetest Main Menu _006](https://f.cloud.github.com/assets/3399154/235805/a6025902-87c2-11e2-9a24-cb4f1d03b2f8.png)

menusplash can be transparent allowing you to see clouds through it like this: http://digitalaudioconcepts.com/vanessa/hobbies/minetest/screenshots/menu1.png
menusplash overrides menubg and the old menubg was removed, re-adding menubg (dirt from minimal) will restore the old menu look.