Rename gplugin-gtk to gplugin-gtk3

Testing Done:
Built and ran the unit tests and gplugin-gtk3-viewer on my dev machine.
Built the debian packages, installed them in a container and ran gplugin-gtk3-viewer via broadway.
Built the fedora packages, installed them in a container and ran gplugin-gtk3-viewer via broadway.

Bugs closed: GPLUGIN-98

Reviewed at https://reviews.imfreedom.org/r/961/