Let right-click toggle between adding rests and notes

Based on a number of discussions over the years where users expressed displeasure over the steps needed to adds rests using the mouse, I came up with a scheme to simplify this: allow right-click to add a rest if you are otherwise in note mode, or to add a note if otherwise in rest mode. I won't claim this is super discoverable, but it does streamline the workflow for mouse users who do learn about it. Eventually we could add a context menu to also allow selecting duration, but I think adding a rest still makes sense as a default action. PR forthcoming.