Fixed initial graying out options in Cube Effect settings

Typically after deselecting "Show caps" option, the associated options "Cap color" and "Display image on caps" are grayed out (triggered by signal). The bug was when Desktop Cube settings were closed and opened again then the options were not grayed out.

Succesfully compiled and tested by deselecting "Show caps" option in Desktop Cube settings, closing the settings window and opening it once again. The options "Cap color" and "Display image on caps" were then grayed out.

REVIEW: 129194