popover: Set pointer motion mask on the popover window

This is not necessary for the popover itself, but helps tooltips
code confine the widget lookup within the popover if the pointer
is inside it, otherwise the widget lookup may turn out wrong for
motion events, starting the tooltip widget lookup from the toplevel
window, mistakenly triggering tooltips on the natural window
descendants (ie. the widget below the popover)

https://bugzilla.gnome.org/show_bug.cgi?id=724785

gtk/gtkpopover.c

@@ -238,6 +238,7 @@ gtk_popover_realize (GtkWidget *widget)
   attributes.wclass = GDK_INPUT_OUTPUT;
   attributes.event_mask =
     gtk_widget_get_events (widget) |
+    GDK_POINTER_MOTION_MASK |
     GDK_BUTTON_MOTION_MASK |
     GDK_BUTTON_PRESS_MASK |
     GDK_BUTTON_RELEASE_MASK |