Text moved by cursor still considered "styled" and thus not saved

Now I understand better why chord symbols and measure numbers weren't responding to cursor changes in my initial fix for #278128: moving elements with cursor keys. Apparently changing the offset isn't enough to force the element to get marked as not styled, and that's true for everything - staff text, title, etc. This means that the reset and set as style buttons never activate, and more importantly, the new offset does not get written to the file, so all that work manually adjusting things is lost.

I think I can fix, if not we need to roll back the original change which I'd rather not.