Implement Gtk Application support

[dahenson]

https://developer.gnome.org/gtk3/stable/Application.html

I want this for a personal project of mine, and I already have some of the code working locally. I will attempt a pull request when I feel comfortable with it.

The biggest change will be the addition of some functions from gio. Feel free to stop me if you think this work will be unnecessary.

[jrick]

Yes, I was going to do it but got sidetracked with other projects. Thanks for taking the time to look into this.