Automatic caching of render and rows methods.
Class variable no_cache is a list of names of methods to not cache automatically. Valid method names for no_cache are 'render' and 'rows'.
Class variable ignore_focus if defined and set to True indicates that the canvas this widget renders is not affected by the focus parameter, so it may be ignored when caching.
register the list of signals in the class varable signals, including signals in superclasses.