Colour handling fixes
- Menus rearranged so "Advanced" is renamed "Palette options"; "Extra click for input focus" moved to the Paint menu.
- The "Highlight background colour" menu option has been removed to simplify the interface.
- To address all the differences of opinion around the artist-friendly palette arrangement, the default behaviour is to not show the confirmation dialogue box plus a further menu option "Rearrange user palettes" (ticked by default). If the latter's unticked, the remapping to artist-friendly (upon Edit palette) will never take place; if it's ticked, the remapping will always take place except when the confirmation box is enabled and the user clicks Default.
- Greyed out palette options when there's not a full/default 256-colour palette, as appropriate.
- Help text updated to reflect these changes. Admin: Submission for the Paint bounty.