Make line grips disappear while adjusting

After putting a line in edit mode and selecting a handle (itself slightly harder than necessary at times, see #35456: slur control points not reliably selectable), it might be nice if the handles disappeared while making the adjustment, to make it easier to see what you are doing. The trick would be to make them reappear at the right moment, I guess.

See http://musescore.org/en/node/51936 for original discussion.