Bring wooden chest fully into the chest system

Re-register most aspects of default:chest and default:chest_locked,
using the technic chests code, so that the wooden chests fit properly
into the sequence of chest types.  This mainly affects the formspec,
which now uses the style of the other chests, rather than the bare style
used by the default mod.
Better chest color button layout

Where possible (which it currently is for the gold chest), don't break
the centering of the player inventory in the chest formspec because
of the color buttons.  Where the color buttons don't fit next to a
perfectly centered player inventory (which doesn't currently occur for
any technic chest), move the player inventory only as much as necessary
to accommodate the color buttons.
Fix mesecon control of forcefield

The code formerly attempted to make the forcefield emitter controlled
both manually and by (inverted) mesecon signal, but the two interfered
with each other.  In particular, a newly-placed emitted would be
informed that it was getting no mesecon signal, and would therefore
enable itself.  Fix this by adding explicit modes for how the emitter
will respond to mesecon signals: ignore them, obey them positively,
or obey them negatively.

The manual control could have been incorporated into this mode setting
by having two "ignore mesecon" modes: always-enabled and always-disabled.
But it seems more useful to have a separate manual master switch, so that
the emitter can be manually disabled without losing the mesecon mode.
So it is now implemented that way.
Tweak mesecon control of forcefield

Drop support for negative mesecon control.  This requires users of
negative mesecon control to invert their mesecon signal externally.

Comment on rationale for the way toggle buttons in formspec are managed.
Genericise can logic

Factor out the logic common to water and lava cans.  Provide it in the
form of a technic.register_can() function, which can be called by other
mods to register cans for other liquids.
Improved can usage

Make the use of cans more like the digging and placement of ordinary
nodes, and specifically make it much closer to the use of buckets.
The main change is that left-click with a can is now only used to take
liquid; placing liquid is now done with a right-click.  This makes the use
of cans a lot less error-prone, compared to the old scheme of determining
the operation based on the type of node pointed to.  Other changes are
that liquid placement is now permitted to replace any buildable_to node,
and the cans obey node protection.
Better reactor meltdown

Reactor `explosion' now replaces the reactor core with a corium source
node.  Corium is a new liquid, which flows a bit like lava, but has
the additional feature of destroying nodes to which it is adjacent.
It also randomly turns into a solid form, chernobylite, which makes an
attractive building block.  It thus gradually melts its way through the
reactor shielding layers; a meltdown gets worse over time if not cleaned
up promptly.

The mechanism for an active reactor core to damage nearby players is
generalised into a "radioactive" node group.  Corium and chernobylite
are radioactive, to varying degrees.  Players receive a varying amount of
damage from a radioactive node, depending on proximity.  Staying outside
a reactor cube is sufficient to be safe from the active core, but not
sufficient to be safe from a melted core.