Extending facet's mechanism of gathering changes
    
    https://fedorahosted.org/freeipa/ticket/2041
    
    Adding option to gathering changes for update from widgets, sections, details facet.
    
    Changes are represented by update_info { fields [] ((field_info)), commands [] ((command_info))  } object.
    
    * On calling get_update_info() method widget, section and facet returns update_info object which represents all changes in nested objects. Thus usually widgets are creating update_infos, their containers are merging them.
    * This object can be then used in details facet update method. In order to use it command_mode = 'init' has to be set. Command mode was introduced to support backward compatibility.
    * command_info consists of command and priority. Priority can be set to specify exact exectuting order of commands. It can be defined on facet level by setting widget's priority. When widgit is creating command_info it should pas its priority to it.