* Added a generic 'histogram' ui control, currently available in new image editor
'scopes' region (shortcut P). Shows the histogram of the currently viewed image.
It's a baby step in unifying the functionality and code from the sequence editor,
so eventually we can migrate the sequence preview to the image editor too,
like compositor.
Still a couple of rough edges to tweak, regarding when it updates. Also would
be very nice to have this region as a partially transparent overlapping region...