Use KNotifications if Frameworks integration is enabled

Use KNotifications if Frameworks integration is enabled

This brings back proper support for KNotify if KDE Frameworks
integration is enabled.

Note that quassel.notifyrc is installed in a different location
if KF5 as opposed to KDE4. If you don't have a system installation
of Quassel, you may have to manually copy that file from data/
to $HOME/.local/share/knofitications5 for KNotify to know about
Quassel.