As outlined earlier, in order to turn the Extensions app into a properly
sandboxed application, we need to split out the extension prefs dialog
and move it elsewhere.
With "elsewhere" being the new Extensions D-Bus service, effectively
turning it into a shell extensions portal.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1106
The dialog that contains the extension's preference widget has become
fairly complex over time, mostly due to the error handling.
It therefore makes sense to move it to a template, just like we did
for the main application window and extension rows.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1106
Now that the org.gnome.Shell.Extensions interface exposes the
disable-user-extensions setting on D-Bus, we can use that instead
of the shell's GSettings.
In a future where we distribute the app separately as flatpak, this
will require one less hole in the sandbox.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1081
