some refactoring

git-svn-id: file:///svn/Scribus/trunk/Scribus@19530 11d20701-8431-0410-a711-e3c959e3b870
#12662: prevent deletion of "Black", "White" and registration color

git-svn-id: file:///svn/Scribus/trunk/Scribus@19531 11d20701-8431-0410-a711-e3c959e3b870