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.
CHANGED: Settings are now stored in versioned sub-directories
ADDED: First-run dialog for migrating settings from a previous version
CHANGED: Settings are now stored as JSON files instead of wxConfig-style INI files
CHANGED: Color settings are now all stored in a separate settings file
CHANGED: The symbol editor and footprint editor now have their own settings files
CHANGED: Color settings are no longer exposed through BOARD object
CHANGED: Page layout editor now uses Eeschema's color scheme
Settings are now managed through a central SETTINGS_MANAGER held by PGM_BASE.
Existing settings will be migrated from the wxConfig format on first run of each application.
Per-application settings are now stored in one class for each application.