SwipeScroll: Only allow starting with the left button

Currently we allow starting swipe-scroll with any mouse button
which is weird from an UI pov and causes inconsistencies with
event handling.

https://bugzilla.gnome.org/show_bug.cgi?id=647186

js/ui/overview.js

@@ -281,7 +281,8 @@ Overview.prototype = {
     },
 
     _onButtonPress: function(actor, event) {
-        if (this._scrollDirection == SwipeScrollDirection.NONE)
+        if (this._scrollDirection == SwipeScrollDirection.NONE
+            || event.get_button() != 1)
             return;
 
         let [stageX, stageY] = event.get_coords();