Side transformation/substitute actions

Side transformations and node substitutions can be implemented with the same infrastructure. There are left/right/center transformations.

From the perspective of defining actions it's useful to distinguish between "replacing" a cell and "inserting between" two cells. Distinguishing between left and right transformations as in MPS while defining an action is not useful. You could specify that an action should be available in the gap before or after a node/cell, but triggering it should be possible from both sides of that gap.

To make the difference clear the action locations should be named before/after instead of left/right.