Panel: sync primary hotcorner hover with Activities button

Since the hotcorner is a reactive actor, and it is over the Activities
button, hovering on it results in a leave-event for the button.
This is not noticeable when opening the overview, as the button is
correctly prelighted, but it is when closing, if you keep the mouse
near the hot corner, as the button is kept in normal state, despite
the mouse being over it.

https://bugzilla.gnome.org/show_bug.cgi?id=645751

js/ui/main.js

@@ -539,7 +539,7 @@ function _relayout() {
         if (!isPrimary && !haveTopLeftCorner)
             continue;
 
-        let corner = new Panel.HotCorner();
+        let corner = new Panel.HotCorner(isPrimary ? panel.button : null);
         hotCorners.push(corner);
         corner.actor.set_position(cornerX, cornerY);
         if (isPrimary)