Ftr/358 grayout goto movement

[bwbohl]

This pull request introduces changes to the go-to menu and partly fixes #358. The number of movements is checked, and the goto Movement and the movement combobox in the go-to measure menus are grayed-out if there is only one or fewer movements.

• [x] SourceView
• [x] VerovioView
• [x] mdiv combobox in View MeasureBasedView