[kcm/kwinrules] Add properties to a rule one-by-one
Change the labels to singular `Property` and close the property sheet after clicking on one item. The behaviour of the sheet is now more similar to a menu, and not so much as a dialog which needs to be dismissed to go on. The idea is to simplify the rule editor workflow and make it more evident to the users. By making the `Add property` close after each selection, the user can see immediately that the property has been added to the rule list so they can edit it. Also use ListView transitions to add visual hints when adding or removing properties, and try to position the new added item into the visible view.
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