Add a new setExclusiveEdge call in the protocol

This can be used to disambiguate the exclusive edge when the anchors are
on a corner (so there  would be 2 candidates)

it's quite quick and dirty mostly to understand if we do want to push
for something along the lines

src/qwaylandlayersurface_p.h

@@ -36,6 +36,7 @@ public:
 
     void setAnchor(uint32_t anchor);
     void setExclusiveZone(int32_t zone);
+    void setExclusiveEdge(uint32_t edge);
     void setMargins(const QMargins &margins);
     void setKeyboardInteractivity(uint32_t interactivity);
     void setLayer(uint32_t layer);