Introduce e_extensible_list_extensions(), which provides extensible
objects access to their own extensions, or a subset of them.
Convert EShellBackend to an abstract EExtension subtype. EShell will
load its extensions with e_extensible_load_extensions(), and then obtain
a list of EShellBackend extensions as follows:
shell_backends = e_extensible_list_extensions (
E_EXTENSIBLE (shell), E_TYPE_SHELL_BACKEND);
Because EShellBackend is abstract, its GType is skipped while traversing
the GType hierarchy to find EShell extensions.
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