kcm/kwinrules: Keep sheet open when adding properties via button

Keep properties sheet open when clicking on the button to add a new property. This allows to add several properties in a row. Clicking on the full row will keep the previous behavior for the simple case, closing the sheet so the new property can be edited right away

Also make the button on each delegate always visible (not only on hover) to be more consistent with current status everywhere else.

Previously the sheet would remain open only after detecting some window properties, which was a very hidden and confusing pattern.