This refactors the GLE base class into an interface, to make it
possible to not depend on GEF at all.
The GLE2 editor part displays and does nothing. It's just an empty shell.
To enable, setenv USE_GLE2 to anything. Unset to remove it.
Change-Id: I7a95b4a1a5a8ddf5a3f18acf7a04b1f9b3439655
59aac85f63