Port to GTK4

[mdosch]

Dear Alexander,

thank you very much for gummi, it is a very useful program. Without gummi using Latex would be too tiresome for me and my documents would not look nice. :)

Would you consider porting gummi to GTK4 to make gummi future proof and assure it will stay around for some more time?

https://docs.gtk.org/gtk4/migrating-3to4.html