app: in GimpToolWidgetGroup, calculate hover widget at button press

In gimp_tool_widget_group_button_press(), explicitly call
gimp_tool_widget_group_hover() before forwarding the event to the
group's hover widget, so that the hover widget gets recalculated.
If a widget is added to the group as a result of a button-press
event, this guarantees that it gets considered as a target for the
same event.

(cherry picked from commit 5b217b3ad4)

app/display/gimptoolwidgetgroup.c

@@ -227,6 +227,8 @@ gimp_tool_widget_group_button_press (GimpToolWidget      *widget,
   GimpToolWidgetGroup        *group = GIMP_TOOL_WIDGET_GROUP (widget);
   GimpToolWidgetGroupPrivate *priv  = group->priv;
 
+  gimp_tool_widget_group_hover (widget, coords, state, TRUE);
+
   if (priv->focus_widget != priv->hover_widget)
     {
       if (priv->hover_widget)