and use it to draw the layer boundary. Remove a lot of stuff
that was there only to draw the boundary before:

- remvoe all layer boundary stuff from the selection code
- remove gimp_display_shell_draw_layer()
- remove enum values GIMP_SELECTION_LAYER_ON,OFF from core-enums.h
- remove all lines calling gimp_image_selection_control() with
  the removed enum values
- remove gimp_layer_boundary()