We're currently only using GimpHighlightableButton in the layers
dialog, which defines its own set of highlight colors. We're going
to use highlightable buttons in the dashboard too, so let's move
the highlight colors to gimphighlightablebutton.h, and give them
standard names. We currently define
GIMP_HIGHLIGHTABLE_BUTTON_COLOR_AFFIRMATIVE (green), and
GIMP_HIGHLIGHTABLE_BUTTON_COLOR_NEGATIVE (red).
(This commit was accidentally dropped from the gimp-2-10 branch; it
should have gone before 40ac4f7bc0f43aee24dc7ae1cf674d1a59612f55.)
