Move keyboard detection tree file to keyboard page

The pc105.tree file from Ubuntu is now included in the GResource that
belongs to the keyboard layout page.

[endlessm/eos-shell#3425]
Port Python keyboard detector to C

This is a straightforward port to C of the keyboard detector code
from Ubuntu's installer.

(Roddy, Philip)

[endlessm/eos-shell#3425]
Incorporate keyboard detection heuristic

Hooks up the dialog box ported in the previous commit to the keyboard
layout selection page. If the user clicks "Add" after the keyboard has
been guessed, it will be added to the list of keyboards on the page, and
automatically selected so that the "Next" button becomes active. If the
guessed layout was already in the list, it will still be selected.

(Roddy, Philip)

[endlessm/eos-shell#3425]