Improvements in definition of the icons.

Improvements in definition of the icons by user.
With this changes, now we can set icon per options/group.

Wiki/Doc:
- To add the icon option:
  * Add attribuite "uix-option-icon" to original select option.
- To add the icon group:
  * Add attribuite "uix-group-icon" to original select group.
  * Note: Icon group will be applied only when CollapsableGroups = False
- To add the collapse icon group:
  * Add attribuite "uix-collapse-icon" to original select group.
- To add the collapse icon group:
  * Add attribuite "uix-expand-icon" to original select group.

Best Regards