Convert TalkatuDemo to a GtkApplication

While this is a requirement for GTK4, it can be done in GTK3, so this targets `default`. I don't know if we want to merge/rebase `gtk4` over it, `graft` it on, or just only apply it to `gtk4` and leave `default` untouched.

Testing Done:
Compiled and ran.

Reviewed at https://reviews.imfreedom.org/r/1260/