Don't trigger screen edge if the pointer is constrained

If the pointer is constrained, the screen edge may push the cursor back,
which will work not as expected. It's also undesired to trigger the screen
edge while the pointer is constrained by some surface, in general.

src/screenedge.cpp

@@ -225,6 +225,10 @@ void Edge::unreserve(QObject *object)
 
 bool Edge::activatesForPointer() const
 {
+    if (input()->pointer()->isConstrained()) {
+        return false;
+    }
+
     bool isMovingWindow = false;
 
     // Most actions do not handle drag and drop properly yet