D21249: Test current filter before setting a new one

[Elvis Angelaccio]

elvisangelaccio added a reviewer: dfaure.

REPOSITORY
  R241 KIO

REVISION DETAIL
  https://phabricator.kde.org/D21249

To: jglogowski, #frameworks, ngraham, dfaure
Cc: elvisangelaccio, ngraham, michaelweghorn, kde-frameworks-devel, michaelh, 
bruns