which gets emitted when the item wants to be redrawn:
- Emit "update" it when any item property changes
- Groups connect to their children and forward "update" for them
- The shell connects to its group of canvas items and exposed
the affected area
- Remove gimp_display_shell_expose_item()
- Move all the shell's item group code into gimpdisplayshell-items.c
- Invalidate the proper area when an item gets added or removed.
- Draw the kept canvas items instead of calling a draw tool function.
- The draw tool now sets its item on the shell.
