gplugin/gplugin

Add GTK4 viewer outline

2021-10-09, Elliott Sales de Andrade
d926130c6d12
Add GTK4 viewer outline

This does nothing since the widget isn't implemented, but it's setup to use GTK4 paradigms.

Testing Done:
Compile and run `gplugin-gtk4-viewer` with the various options, and some temporary debug prints.

Reviewed at https://reviews.imfreedom.org/r/992/
The following individuals have contributed to this project.
Ankit Vani <a@nevitus.org>
EionRobb <eion@robbmob.com>
Elliott Sales de Andrade <quantum.analyst@gmail.com>
Gary Kramlich <grim@reaperworld.com>
ivanhoe <ivanhoe@fiscari.de>
Jason Scherer <schereja@gmail.com>
John Bailey <rekkanoryo@rekkanoryo.org>
Mike Ruprecht <cmaiku@gmail.com>
Olaf Hering <olaf@aepfle.de>
Richard Laager <rlaager@pidgin.im>
Sorvival <sorvival@tutanota.com>