so that "desert stone" won't select "desert stone block"

closes #165

Markers are part of the UI (`worldedit_chatcommands`) and do not belong in `worldedit`.

Additionally, this commit makes the command execution more modular,
so it is e.g. possible to implement something to disable the
many nodes confirmation after setting a checkbox in the formspec.

Also fixes a crash when detonating TNT near them though this is
not our fault and the proper fix is pending in minetest_game.

wtf?

continuation of 45acf09e4a

This happens when you don't test.

In fact, allow all commands that are compatible by the brush:
the ones that expect exactly pos1 to be set

fixes #187

Though right-click currently doesn't work due to an engine bug.

Can't be bothered to do the rest. This stuff should be
rewritten in a modular fashion anyway.

This matches the behaviour before introduction of the vmanip code.

Since the code has two arrays of the area before and after,
the copy order is totally irrelevant.
Of course, this doesn't apply to metadata and we still need
to do that backwards.

Currently no schematic format makes use of these, but that
may change so do some future proofing.

The underlying issue is the size detection worldedit.allocate does
and can't be fixed or worked around in a simple way.