Measure Merge

A right-click in any measure stack allows us to merge this measure with the following one on the right, via the menu <popup> | Measure #N | Merge on right (which is disabled for the last measure of a system).

It is a convenient way to remove all the intermediate barlines, if any, and merge the two measure stacks into a single one.

TODO: provide a concrete score example.

We can undo/redo this operation.