pep8 on backend_gtk.py

Two line-length violations on long urls in comments are left.
pep8 clean up in backend_gdk
removed un-needed Gcf import

changed imports to _backend_bases to avoid implicit Gcf import
split backend_gcf into two parts,
one which includes Gcf and one that does not.
pep8 on backend_gtkagg.py
split backend_gktagg.py in to two parts

_backend_gtkagg.py has no dependence on `Gcf`, backend_gtkagg.py
depends on `Gcf` and maintains reverse compatibility.
updated exclude list