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.

doc/python_api/sphinx_doc_gen.py

@@ -1054,7 +1054,6 @@ context_type_map = {
     "texture_slot": ("MaterialTextureSlot", False),
     "texture_user": ("ID", False),
     "texture_user_property": ("Property", False),
-    "uv_sculpt_object": ("Object", False),
     "vertex_paint_object": ("Object", False),
     "view_layer": ("ViewLayer", False),
     "visible_bases": ("ObjectBase", True),