Adds a mesh model that appears when a chest is opened. The chest
stays visibly open as long as the player keeps it open. When the
player closes the formspec, the chest returns back to the closed
shape. While opening and closing, a sound plays.

A second person inspecting the chest will trigger a second sound
open. However, only after the last player closes the chest, does the
chest actually visually close and is the sound close played. This
keeps mesh updates to a minimum.

While it's possible that a server shutting down may cause chests
to remain open, this does not affect the chests' working matter,
and opening or closing them should fix them.

Old chests are converted to the new style by LBM. I previously
had them converted on open but this was unreliable, and LBMs
don't have that problem.

Open chests cannot be dug up. This prevents people from keeping
a chest open and digging it out as well, since closing a chest
would place a chest back (swap) at the spot. We could protect
against this, but it still messes up the client and causes a lot
of "missing node inventory" error messages otherwise. It's
unlikely but possible that a player lagging out causes a chest
to stay "open" and thus unremovable by digging, but there are
other ways of dealing with that - a server restart fixes that
issue.

If the lid of the chest is obstructed, the sounds continue to play,
but the lid isn't opened. Obstructed means that a node is present
above the chest lid, however, we ignore several node types like signs,
torches (not 3d) and wallmounted nodeboxes (typically signs) since
they don't pose any major obstruction in almost any case, and are
typically found above chests. Additionally, the selection box of the
opened chest does not include the lid, and so one can still interact
with e.g. a sign behind an open lid.

Due to the fact that chests now have 7+ textures, we can no longer
use materials (limit: 6) to texture the chest, and so there is now
a single UV mapped image that applies both to open and closed chests.
While this does mean texture pack makers need to create it, this
is extremely simple and consists of a simple cut'n'paste over the
template and should be really easy to do. Only one texture file is
now then used for both open and closed chests.