[kwinrules] Properly setting the value of the activity combo box

Summary:
The activities combobox content is loaded asynchronously,
we are storing the configured value for the window rule
until the activity list is loaded into the combo box.
And then, we set that item as the current one in the combo.

Reviewers: #kwin, mart

Reviewed By: mart

Subscribers: luebking, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D2851