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/.

.gitignore

@@ -12,7 +12,8 @@ tags
 /games/*
 !/games/minimal/
 /cache/
-/textures/
+/textures/*
+!/textures/base/
 /sounds/
 /mods/*
 !/mods/minetest/