WIP: First attempt at making rendering tests pass when running headless.

Rendering tests are failing in CI tests due to the headless nature of the servers (https://bugs.launchpad.net/inkscape/+bug/1806878). This "fixes" the tests by creating an executable where InkApplication is derived from Gio::Application rather than Gtk::Application. As I'm not a CMake Guru, I may have done this in a less than optimal way.