Use gtk_widget_set_visible for everything

gtk_widget_show and gtk_widget_hide are deprecated in GTK 4.10. However,
gtk_widget_set_visible has been around forever so we just need to move to that.

I didn't test all of these and there are probably some that can still be
removed, but most of those are in code that is going to get refreshed in the
future so I didn't bother for now.

Testing Done:
I tested the join chat, add chat, and add buddy dialogs, as well as the contact added notification.

Reviewed at https://reviews.imfreedom.org/r/2430/