Disambiguate ‘Spot’ strings for translation

Adds a ‘msgctxt’ for the ‘Spot’ autofocus metering mode string
in the Exif editor, so that it can have a different translation
than other ‘Spot’ (colour) strings. Also changes the ‘Spot’
string in the palette editor to ‘Spot color’, to make it
clearer what the checkbox does.

BUG: 410339