Composer improvements

This pull adds a number of handy features to the composer:

- A new edit menu, with cut/copy/paste, undo/redo, and delete item
- Select next item above/below command in edit menu
- CTRL - click a composer item to select an item below it (fixes #4358)
- holding shift while pressing cursor keys results in large item movements

Some of this is stretching my current Qt abilities, so I'd appreciate a review prior to pulling!