Simply clicking once on a block should not place it

It is very confusing when people just click on a block, either in a function or list comprehension, or from the toolbox, and a copy of that block gets placed in the workspace. We should wait for some minimum drag distance before treating this as a drag.

Reproduce the behavior by clicking blocks in a function, list comprehension, and toolbox without dragging. Locate the block interaction handling that distinguishes clicks from drags, then verify that a click does not place a copy while dragging beyond the chosen minimum distance still does.

Written by the indexing model from the issue text.