app: fix visibility of the color picker tools info window

Since commit 867b1f7e the window did always pop if it was set to
visible once, even if closed and "Use info window" disabled.

Fix this by adapting the show logic to the fact that we now keep the
widget alive across images/displays.

app/display/gimptoolgui.c

@@ -391,6 +391,21 @@ gimp_tool_gui_get_vbox (GimpToolGui *gui)
   return GET_PRIVATE (gui)->vbox;
 }
 
+gboolean
+gimp_tool_gui_get_visible (GimpToolGui *gui)
+{
+  GimpToolGuiPrivate *private;
+
+  g_return_if_fail (GIMP_IS_TOOL_GUI (gui));
+
+  private = GET_PRIVATE (gui);
+
+  if (private->overlay)
+    return gtk_widget_get_parent (private->dialog) != NULL;
+  else
+    return gtk_widget_get_visible (private->dialog);
+}
+
 void
 gimp_tool_gui_show (GimpToolGui *gui)
 {