WIP: general: Make property extensions gtk version independent

Nautilus property extensions to add property pages to the property dialog was providing a GtkWidget to be modified by extensions.

This makes the extension need to target a specific gtk version, which with the new gtk versioning might be hard to provide, and it's quite a bad practice since it requires everyone to be on top of any gtk update. This is currently holding the work for porting Nautilus to gtk4, since the Totem extension depends on us having the same gtk+ version, which is unlikely.

This work makes the extension for providing property extensions not depend on gtk by providing a plain struct to be filled with data that later on Nautilus will layout on the UI.

See #276 (closed)