New Navigation Commands to handle MMRests appropriately

Mike320 mentioned that a crash occurs when issuing [next system] when a score ends with a multi-measure rest. Upon further investigation, the issue is bigger than that. If the user is within a multi-measure rest and then issues the command, also a crash occurs. There was no special code dealing with MM-rests.

This has been fixed (PR shortly) and improved to handle MMRests, but also [Top Staff] and [Go To First Empty Trailing Measure] will have no problem with these. Additionally, and here's the improvement: before, the behavior was as with how [next/previous measure] works: if moving into a MMRest measure, [next/previous measure] doesn't actually select the Multi-Measure Rest when coming in from Note Entry. This in turn doesn't shift the view of score. That means [go to first empty trailing measure] + [next/prev system], if it moved to a MM Rest, didn't actually move the score's view point to that measure. That has been updated now. Thanks again to Mike320 for posting about the MM-rest issue.