When loading indexed images, the image type is not obvious at all
(basically only reference is in the title bar). The main issue is that
if you don't realize it when editing, GIMP appear broken when the
expected colors don't appear on canvas.
Commit e48c2394 was a first step by showing various color widgets with
out-of-gamut warnings contextually. This additional commits will also
allows color selection for painting tools (i.e. foreground and
background colors) to be done within the image palette by default. This
way, the fact that this image impose working with limited color palette
is obvious as soon as you try to edit colors.