Implement new save-changes-dialog mockups
We have updated designs for the Save Changes Dialog.
Notes
- You can't subclass
GtkMessageDialog
in GTK 4, so it's implemented usinggtk_message_dialog_new()
directly - You can find the implementation at https://gitlab.gnome.org/GNOME/gnome-text-editor/-/blob/main/src/editor-save-changes-dialog.c