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Add a portal-like Extensions D-Bus service

Florian Müllner requested to merge fmuellner/gnome-shell:extensions-portal into master

Depends on and includes the changes from !1081 (merged) which has a pending freeze break request.

Quoting the commit message with the rationale:

Similar to the previously added org.freedesktop.Notifications proxy,
this exposes the org.gnome.Shell.Extensions API and forwards any
request to the real implementation in gnome-shell.

The motivation differs though: We want to be able to package the
extension app as flatpak and distribute it separately, but the
extension prefs dialog is hard to impossible to sandbox:

 - filenames need translating between host and sandbox, and we
   can only do that in some cases (serializing/deserializing
   extensions), but not others (extension settings that refer
   to files)

 - system extensions install their GSettings schemas in the system
   path; the best we can do there is assume a host prefix of /usr
   and set GSETTINGS_SCHEMA_DIR in the flatpak (eeks)

 - extensions may rely on additional typelibs that are present on
   the host (for example because gnome-shell itself depends on
   them), but not inside the sandbox - unless we bundle all of
   gnome-shell's dependencies

 - if gjs/mozjs differ between host and sandbox, extensions must
   handle different runtimes for the extension and its prefs

And all those issues occur despite a very permissive sandbox (full
host filesystem access, full dconf access, full org.gnome.Shell
access (including Eval()!)).

This new service will give us an alternative place for handling
the preference dialog:

 - it runs outside of gnome-shell process, so can open windows

 - it runs on the host, so the extension's prefs get to run
   in the same namespace as the extension itself

That is, the service will provide portal-like functionality (albeit
not using the org.freedesktop.portal.* namespace, as extension
management is an inherently privileged operation).

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