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.
The mechanism here is simple but hard to explain without leaning heavily
on object-oriented jargon. Consider this a rough draft. Illustrations
would certainly help clarify.
