workspacesView: Consider workspaces-only-on-primary when scrolling

It is odd to switch workspaces using the scroll wheel when the pointer
is on a monitor without workspaces, so only handle scroll events on
non-primary monitors when workspaces-only-on-primary is disabled.

https://bugzilla.gnome.org/show_bug.cgi?id=766883

js/ui/workspacesView.js

@@ -674,6 +674,11 @@ const WorkspacesDisplay = new Lang.Class({
     _onScrollEvent: function(actor, event) {
         if (!this.actor.mapped)
             return Clutter.EVENT_PROPAGATE;
+
+        if (this._workspacesOnlyOnPrimary &&
+            this._getMonitorIndexForEvent(event) != this._primaryIndex)
+            return Clutter.EVENT_PROPAGATE;
+
         let activeWs = global.screen.get_active_workspace();
         let ws;
         switch (event.get_scroll_direction()) {