Icon name fix
Revamp tool settings dock

Remove dual color button. The color dock is now the primary way
of setting colors. The color dialog can still be called up by double clicking
on the brush preview (or via context menu.)

Also, add custom styling to dock headers.
Icon refactoring

Split icons into "theme" and "builtin" categories.
Theme icons are used as fallbacks for system theme icons
and builtin icons are always included in the executable.