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.
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