Show a warning when editing curves created for a different colorspace

It's the best we can do now; we should label the curves, and only
reset the real color curves, and keep the virtual curves, but the
file format and the structure of the multichannel filter don't
really support that.

This breaks string freeze :-(

CCBUG:397612
CCMAIL:kimageshop@kde.org
CCMAIL:kde-i18n-doc@kde.org