Finish port of keyboard section to introspection



This patch finishes the port from GTK2 to GTK3 begun by manuq for the keyboard section of the control panel and addresses issues raised in SL #4492 and #3926. It is dependent upon dnavraez's Sugar extension [1].

A few comments:
(1) in model.py, xklavier has been replaced by XKL; this required a few syntax changes. this fix prevents selection of the keyboard section from crashing Sugar (by mixing gtk2 and gtk3)
(2) the old code did not handle language selection properly (this is fixed in view.py LayoutCombo by retrieving language from ~/.i18n as per cpsection/languages
(3) there is some pep8 clean up interspersed

[1] dnarvaez/sugar-toolkit-gtk3@a533886