Play player_damage.ogg when recieving damage and additionally play player_falling_damage.ogg when recieving falling damage

This reverts commit 649dca4d.

Now Environment::clearAllObjects() unloads unused blocks in an interval
defined by max_clearobjects_extra_loaded_blocks (default 4096).

This fixes the bug where noMenuActive() keeps returning false after
closing a menu until the mouse is moved, rendering the keyboard
unusable (the_game calls input->clear() every frame when
noMenuActive() is false).

This should fix #730.

Makes the result compatible with the minetest.get_inventory(location) param.

On the lua side, notably minetest.env:<function>(<args>) should now
be replaced by minetest.<function>(<args>).
The old way is and will stay supported for a long time.

Also:
Update and clean up lua_api.txt (by celeron55)
Move EnvRef to lua and remove add_rat and add_firefly (by kahrl)
Add separate src/util/CMakeLists.txt, other minor fixes (by kahrl)