Add a canvas item group for previews, and a small preview infrastructure
to GimpDrawTool, and put the transform preview into the preview group,
which is below all guides, grid and layer boundaries.
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.
Michael Natterer