To allow a much easier porting to the new layout, the keyboard
panel is now a template class. That has various implications on
the code organization:

 - The keyboard-shortcuts.c was responsible for filling the shortcuts.
   Because it relied on the GtkBuilder of the panel, most of its code
   was moved to the CcKeyboardPanel class.
 - The unused code from the keyboard panel class had to be removed in
   order to make it work again.
 - All the hash tables and widgets are now part of the CcKeyboardPanel
   structure.
 - The interface elements have a single entry point.

https://bugzilla.gnome.org/show_bug.cgi?id=769063