Vega-strike-engine-source: Migrate from GTK2 to GTK3

[nabaco]

We use GTK+2 in our code, and I propose that we migrate to GTK+3.
This will allow us to use both more modern libraries, with better support, and will ease compilation on Windows, where getting GTK+2 is somewhat an issue.

Related to #53

For information see here:
https://developer.gnome.org/gtk3/stable/gtk-migrating-2-to-3.html

[nabaco]

Can't assign to DMJC, probably since he's still not listed as a contributor in this repo, but he's taken this task upon himself, and is nearing completion.

[BenjamenMeyer]

Yeah you have to be an org member to get assignments

[Loki1950]

Stupid question shall I add him or will you @BenjamenMeyer he's got it done though it does not write to the config file asked him to post a link to his repo on gitter

[DMJC]

I've sent a pull request through.