Extra sound effects

This adds various sound effects to minetest_game, including:

- Open and close sounds for doors, taken from the Home Decor mod by VanessaE. A sound file for gates is also included, to be used with fence gates when such is added.

- Water and lava sounds. They are looped on any liquid source that causes flowing liquid (most noticeable with non-finite fluids). Note that they're only triggered for sources that don't touch other sources, otherwise a leaking lake bombards the server and the player's headphones with noises.

- Interaction sounds for chests and furnaces. A chest sound is played when opening a chest, while the furnace sound is triggered when a furnace becomes active. A sound for the workbench is also included, to be used once a workbench is included.

- Eating sound, triggered when using an apple or bread.

Please test, post your opinion, and merge when everyone's ready. Water sounds should be tested specifically, to see if they behave correctly in all circumstances. Original sound sources (note that most were modified):

http://freesound.org/people/Slanesh/sounds/31768/
http://freesound.org/people/j1987/sounds/106116/
http://www.pdsounds.org/sounds/water_over_rocks_as_tide_recedes
http://www.pdsounds.org/sounds/squeaky_door
http://www.pdsounds.org/sounds/rusty_metal_door_clasp
http://www.pdsounds.org/sounds/wood_and_cutlery
http://www.pdsounds.org/sounds/crunching_munching_grinding