Choose one style of interacting with GTK+ widget instance attributes and refactor code accordingly

[elbenfreund]

GTK+ allows for various ways to interact with instance attributes. We should pick one for consistency and refactor the codebase accordingly. This should also be added to the docs and styleguide.

The GTK+ project does not seem to have a recomendation about which style to use.
The following is from #gnome-hackers @ freenode (2017-06-30)

(14:41:29) elbenfreund: there :) I am a bit unsure about current GTK+ best practices interacting with widget instance attributes. I know about *.props.ATTRIBUTE as well as the corresponding setter methods (see also https://stackoverflow.com/questions/15180320/editing-gtkwidget-attributes-properties)
(14:41:35) muelli hat den Raum verlassen (quit: Ping timeout: 182 seconds).
(14:42:32) elbenfreund: what I am not clear about is what is the recommended thing to use when both are present.
(14:42:48) ebassi: elbenfreund: Whichever makes more sense in the code base you're working on
(14:43:17) ebassi: props.foo will call g_object_set_property(), which will likely cally set_foo()
(14:43:42) ebassi: Which one you use is entirely up to your taste
(14:43:53) elbenfreund: my usecase is a fresh codebase that has the luxury of trying to establish best practices from the get go :)
(14:44:13) ebassi: Then you get to pick one :-)

[elbenfreund]

My personal preference would be to use the WIDGET.props.ATTRIBUTE interface as the most pythonic way to do this.

[jtojnar]

Sounds good. Maybe it could be also enforced automatically by pylint custom checkers.