UV Sculpt: improve tool-system integration

In 2.7x UV sculpt was a kind of sub-mode
(a toggle with it's own key-map & drawing code).

Move this to an operator that uses the tool-system,
this simplifies internal logic, especially brush selection
which now matches sculpt and other paint modes.

- Remove toggle used to enable uv sculpt.
- Expose the brush, which was already used but there was no way to
  select different brushes.
- Make UV sculpt use paint paint tool slots
  (using brushes how all other paint mode currently do).
- Move UV Sculpt keymap to the tools keymap.
- Remove Q to toggle UV sculpt mode,
  S/P/G keys to switch tools.

source/blender/editors/uvedit/uvedit_draw.c

@@ -479,14 +479,12 @@ static void draw_uv_shadows_get(
 }
 
 void ED_uvedit_draw_main(SpaceImage *sima,
-                         ARegion *ar,
                          Scene *scene,
                          ViewLayer *view_layer,
                          Object *obedit,
                          Object *obact,
                          Depsgraph *depsgraph)
 {
-  ToolSettings *toolsettings = scene->toolsettings;
   bool show_uvedit, show_uvshadow, show_texpaint_uvshadow;
 
   show_uvedit = ED_space_image_show_uvedit(sima, obedit);
@@ -509,9 +507,5 @@ void ED_uvedit_draw_main(SpaceImage *sima,
     else {
       draw_uvs_texpaint(scene, obact, depsgraph);
     }
-
-    if (show_uvedit && !(toolsettings->use_uv_sculpt)) {
-      ED_image_draw_cursor(ar, sima->cursor);
-    }
   }
 }