Make task manager apply slim margins if defined by default and extend to borders

This moves applying the margins to applets from the panel view to the panel containment; on a horizontal panel, the left and right margins are still applied in the view because they are applied only one time to the borders, while the top and bottom borders are applied in the containment applet-wise, unless the applet specified that they don't want to have margins.

Needs plasma-workspace!304 (merged) To be used with https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/60 !94 (closed)