Add cloud menu background by Krisi, configurable with the menu_clouds option.



Also add a menuheader, menusplash, menufooter_clouds, and allow HD menu footers.
And finally don't git-ignore textures/base/.
Screenshot:
![Minetest Main Menu _010](https://f.cloud.github.com/assets/3399154/255257/555132e0-8c08-11e2-97c5-b5ca9f52545c.png)
And with menu_cluds disabled:
![Minetest Main Menu _011](https://f.cloud.github.com/assets/3399154/255261/623a6e0e-8c08-11e2-99b9-54c6ee8ecd01.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.