Turn extension-prefs into an app

Merged Florian Müllner requested to merge fmuellner/gnome-shell:appify-extension-prefs into master

GNOME Software's extension support isn't great, and better off in a specialized app. The plan is to turn gnome-shell-extension-prefs into this app.

This is the first part of implementing the corresponding mockup, and deals with "appifying" the tool.

Follow-up merge requests will change the row appearance and add support for deinstallation and updates.

Related: #1968 (closed)

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