List comparer: only enable Compare button when at least one article in each source list

git-svn-id: svn://svn.code.sf.net/p/autowikibrowser/code/AWB@10477 5227b50d-9349-4574-b055-4b0a7525e11c
List comparer: only enable "Clear all" button when something to clear

git-svn-id: svn://svn.code.sf.net/p/autowikibrowser/code/AWB@10478 5227b50d-9349-4574-b055-4b0a7525e11c
List comparer: only enable "Use list", "Save list" when there's something in the list

git-svn-id: svn://svn.code.sf.net/p/autowikibrowser/code/AWB@10479 5227b50d-9349-4574-b055-4b0a7525e11c