Port about dialog to GTK4

Testing Done:
Opened the dialog, clicked the various options.

There is currently a warning about `markdown_link_target`, but that seems to be a bug in Talkatu (and the links all mush together, which I think is the same problem.) Also, strangely, the dialog sometimes opens exactly as wide as the bottom button box, but sometimes it opens a little bit wider (not sure if it's also Talkatu wrapping causing minor size request changes, or something else).

Reviewed at https://reviews.imfreedom.org/r/1649/