some refactoring of extended image properties dialog

git-svn-id: file:///svn/Scribus/trunk/Scribus@19538 11d20701-8431-0410-a711-e3c959e3b870
remove unnecessary condition check (already checked one level above)

git-svn-id: file:///svn/Scribus/trunk/Scribus@19541 11d20701-8431-0410-a711-e3c959e3b870