Beam edit not fully undoable

Ubuntu 14.04, GIT commit: 0ba6a8b

1) create two beamed eighth notes
2) edit the beam (double click, drag handle)
3) undo
4) select the measure and transpose an octave

Result: beam does not move with the notes, so you get either super long or super short stems.

This is related to #50006: Grace notes incorrectly presented upon change, or undo of it - it's part of the second problem described in the original report. I have a fix for this I'm making as part of the same PR. It's simple Beam::editDrag() sets the generated & user modified properties in a non-undoable way. I don't really understand why the editDrag functions don't use undoChangeProperty, but they don't. Instead, they have static members to remember the original value, and then call undoPropertyChanged() in endEdit(). So I guess I will do the same.