Consistency in keyboard modifiers on drag operations

Recently, a number of discussions have pointed to consistency / usability issues with the way drag operations are interpreted in MuseScore, particualrly with respect to what keyboard modifiers have what effect.

- Users often expect plain drag to select rather than drag the canvas. They are annoyed that it is so easy to drag the canvas, and they can't figure out how to select.

- Shift+drag on the canvas drags freely, but on a score elements contrains vertically

- Ctrl+drag is how you keep a multiselection when initiating a drag operation, but it also constrains the drag horizontally

Seems we've discussed other aspects of this recently, but usually as tangents to other topics. I think it is worth focusing these discussions here to see if maybe a good set of guidelines can be developed that we could consider adopting.