We turned of the modal-ness of the activity configuration dialogue
so that we do not block plasma (https://git.reviewboard.kde.org/r/126945/)
but this introduced a problem when the dialog is opened from the kcm.

This patch allows creating the dialog both as modal and not.
Plasma uses the non-modal version, KCM uses the modal one.

Now, there is a question whether this is smart to do from KCM as well?