cherry-pick from github.com/minetest/minetest_game> Creative: Add global...

cherry-pick from github.com/minetest/minetest_game> Creative: Add global variable creative.formspec_add

This pull adds a new global variable called creative.formspec_add
that will allow mods to add to the creative inventory screen
without the need to fork the mod altogether.  Simple solution
that works already for inventory_plus' BACK button