Expose MuseScore::endCmd() to plugins

MuseScore::endCmd() needs to be called if a plugin does some work outside onRun: (beeing triggered by the user at a later time). I added a wrapper to MuseScore::endCmd().

About mscorePlugins.cpp line 422: This was commented out by Lasconic about a year ago. Since I don't know why, here's the reason I put it back: With this line a score is immediately updated and marked dirty (if needed) after a plugin was triggered. Without this line the user has to click inside the score window first and the plugin even seems to need an extra call to curScore.doLayout().

I also updated the break.qml plugin which seems to be working now.