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
3 jobs for new-properties-extension in 7 minutes and 31 seconds (queued for 3 seconds)
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Status Job ID Name Coverage
  Test
passed #50663
flatpak

00:06:27

 
  Deploy
passed #50664
review

00:01:03

manual #50665
allowed to fail manual
stop_review