Implement remaining config panel functionalities

[pedrogit]

• Add unset properties #24 (high)
• Add a SELECT component for Add item mode #25 (high)
• Add a control for permissions #26 (add/delete items to a user list from the user list) (high)
• Implement default from named function #27 (item_template) (high)
• Implement control to change owner #28 (low). A SELECT component based on values from another list (user list)
• Restrict the scroll bar to the items parts (not the configuration panel) #29 (high)