Replace combobox with palettes on Language cpsection

I cannot find the ticket associated with this bug, but the comboboxes
do not work properly with touch and the long list of language and
country-variants is almost impossible to use.

This patch does two things: (1) separates language selection from
country selection, thus making the language list less unwieldy; and
(2) replaces the combobox with a palette of menus.

The design has been discussed and reviewed [1, 2].

[1] http://wiki.sugarlabs.org/go/Features/Replace_combo_box_in_language_panel
[2] http://lists.sugarlabs.org/archive/sugar-devel/2014-March/047362.html