extensionSystem: Support locking down extension installation
Currently extensions can only be locked down completely by
restricting the enabled-extensions
key via dconf.
This is too restrictive for environments that want to allow users to customize their system with extensions, while still limiting the set of possible extensions.
To fill that gap, add a new allow-extension-installation
setting,
which restricts extensions to system extensions when disabled.
As the setting is mainly intended for locking down by system administrators, there is no attempt to load/unload extensions on settings changes.