* Move common code into Tab parent class
* The Tab init now accepts creation with name, child_widget and tab_label but
will still accept the 'oldstyle' contructor to keep backwards compatibility
with 3rd-party plugins.
* Create namedtuple TabWidget with widget, format func and status_keys. The
TabWidget are stored in a tab_widgets dict making it easier to create, save
and update large numbers of gtk widgets.