layer-shell: add set_exclusive_edge

When the anchor bitfield is set at a corner, such ar top|left, bottom|right and there is an exclusive_zone set, it will have no effect, as is not possible to infer what really the desized edge is: top or left.

Make it explicit adding a separate call set_exclusive_edge, which must be an edge included in current anchors

The plasma shell needs that as it can have panels aligned on a side of the screen, but still require maximized windows to take to account the geometry of the panel