Migrate from GTK 2 to an upstream supported toolkit

[bgermann]

Please consider porting the GUI to a different toolkit. GTK 2 is end of life and Debian wants to get rid of it. The successors are GTK 3 and GTK 4.

[speed47]

That would be good, yes. At some point upstream started to work on it, but they have gone silent again for 1+ year now, so I don't know if we'll hear again from them.

This move requires a lot of work, and I'm not extremely familiar with GTK so it would/will take hours on my side, and my free time is sparse. If anybody wants to jump in, that would definitely help.

[speed47]

in the meantime, the AppImage distribution will continue to work, even if/when OSes drop GTK2 support, so this doesn't directly threatens the availability of dvdisaster on recent OSes, even if obviously, that would be great to use a still-supported GTK version.