Some medium-impact improvements to composer

I've split my original pull request #599 into a number of separate requests. This second one consists of some medium-impact changes as it adds a new 'Edit' menu to the composer and contains some new strings. The changes themselves should be harmless (although the diff for 3eb0ac0 makes it look much more intrusive than it actually is).

This request adds:
- A new edit menu in composers with cut/copy/paste/paste in place/delete items
- Moves undo/redo to the edit menu
- Adds new actions for "Select All", "Select None" and "Invert Selection"

Again, not sure if these can be accepted into 2.0, if not I'll hold off till 2.1.