The diff mode (option in left toolbar) was never a differential display mode.
It is actually an option to force the alpha channel (opacity) of layer colors
to a given value.
So change the name and tool tips to force opacity mode, and make the opacity
parameter adjustable in preferences.
Gerbview: cleanup: remove a old and dead code.
Remove some useless spacers, and do not force some widgets to have a to
large size (by removing empty areas).
Work in progress: one panel is still too big.
jean-pierre charras
Jeff Young