Add toggledecay command

[Argmaster]

Adds a command for toggling decay of leaves and branches.
Allows editing clipboard and selection.

/toggledecay selection on
/toggledecay selection off
/toggledecay clipboard on
/toggledecay clipboard off

A pitfall no. 1 - since pasting with WE updates the blocks mid-pasting, decayable blocks will decay because the pasted structure was incomplete when the blocks were updated. This has to be addressed separately.

A pitfall no. 2 - since the toggling is done in a blueprint, toggling decay on a selection will cause block updates and thus will decay blocks that do not have proper support.

Resolves: #2505