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because constructing the region can be expensive. Also make sure not
to use g_object_set() if nothing would change to further reduce
"update" emissions.

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

because each item has a shell now. Keep the parameter in the virtual
functions though because that saves a lot of code in the subclasses.

and use it in all tools which have groups.

and pass it to all constructors. The GimpDisplayShell is needed
because items are going to become more powerful soon.

- Add signals GimpImage::guide_added(), removed() and moved()
- Remove singal GimpImage::update_guide()
- Adapt core code to emit the new signals instead of update_guide()
- Have the shell connect to the new signals and update guide canvas
  items as needed
- Remove gimp_display_shell_draw_guides()

- Add GimpImage signal "sample-point-moved" and emit it when needed
- Let the shell connect to the sample point add, remove and move signals
  and update the canvas items accordingly
- Remove gimp_display_shell_draw_sample_points()

which is a group that keeps a mapping between arbitrary pointers and
canvas items. No MVC whatsoever yet, just a simple hash table.

For consistency Use a static GParam array for the return values
in all plug-ins.

God knows how many other plug-ins have this.

and add it to gimpcanvassamplepoint.c, it doesn't need to be public
any longer.

and remove gimp_display_shell_expose_guide() and _sample_point().

and use it in the color tool. Also rename
gimp_draw_tool_add_guide_line() to gimp_draw_tool_add_guide().

and draw all sample points in gimp_display_shell_draw_sample_points().

and use it in gimp_display_shell_draw_sample_point().

by using gimp_draw_tool_add_guide_line() for the active guide too.

so the dropped guide is highlighted. Also don't uselessly stop/start
the draw tool in each oper_update(), only do so when the display
changes.

and draw the guides in gimp_display_shell_draw_guides().

And don't call any shell draw functions directly any longer.