Fix Linux support

[Menci]

This PR fixes #43.

Technical details: The original code loads GTK+ inside the Unity process. It causes the gtk_init function to hang in a deadlock (See the backtrace here). I separated the GTK+ related code to a new executable (I'm not sure about Unity's directory structure so I bundled the executable in the library) and run it in a separated process (pass input with argv and get output from pipe()-d stdout) when opening a dialog.

[thinkallabout]

+1 can this get merged in?

[HeySora]

+1!! really useful PR

[JohnGames]

I have implemented this in my fork as it fixes the bugs, thank you!