Refactor widget classes to use Gtk.Template.Child()

[ggalan87]

As shown in https://belmoussaoui.com/article/5-how-to-create-a-gtk-application-using-python-part-1 widget children can be found by reflection using Gtk.Template.Child() , rather than traversal of the tree using the utility function that is used here:

py-producers-dashboard/dashboard/widgets/golivepage.py

Line 40 in 5a36f91

self._message = utils.get_descendant(self, 'message_textview', 0)

We should refactor the code as such.

PS: Thnx to @aggalex who mentioned such syntax in his fosscomm presentation

[aggalex]

I feel flattered :)