Enable Delete key to remove entries (#40)
Based on #40. This change adds delete key support to the data composition and audio composition dialogs.
I am not too familiar with GTK+, so my code might not be the best-practice way to listen for a specific keypress.
Testing items for both data composition and audio CD:
- delete key does nothing when there are no items
- delete key does nothing when there are items but nothing selected
- delete key can remove a selection of a single item without removing any unselected items
- delete key can remove a selection of multiple items without removing any unselected items
- when multiple compositions are open, delete key only affects the active tab