Instrument menu: make instruments more easily movable

Currently, you have to select the instrument, then repeatedly click the arrow buttons in the middle to move something around in the list. This can be very tedious for adjusting the order for large scores, especially when adding more staves to the already large instrumentation.

I'm suggesting allowing each instrument box to be draggable, in the same way you can drag and arrange palettes in the palette menu. This should cut out some time and make the re-ordering process a lot easier. It would also be nice to allow multiple boxes to be selected at once so you can move entire instruments sections/groups at the same time.

It would be great to also add the ability to drag and drop instruments directly from the selection list on the left into the list on the right.