Exult Studio has been ported to GTK+3

[Dominus]

In the last fewweeks ydragon Baroque put in a lot of work into Exult:
- ported Exult Studio to GTK+3
- fixed a close on exit freeze of ES
- fixed duplicate file list of ES on case sensitive OS
- got rid of the X11 dependency of ES on linux and macOS

All except the last one passed Marzo's review the last two days and were accepted into our code.

The Windows snapshot got already updated and as soon as the last one gets accepted there will be Exult Studio snapshots for macOS available.

Let us know what you think!