Move a lot of miscellaneous code related to extensions into a new module

Move a lot of miscellaneous code related to extensions into a new module

ExtensionUtils is a new module that has a lot of miscellaneous things related
to loading extensions and the extension system put into a place that does not
depend on Shell or St.

Note that this will break extensions that have with multiple files by replacing
the old uuid-based importer with an object directly on the meta object.

https://bugzilla.gnome.org/show_bug.cgi?id=668429