Replace combobox with palettes on Language cpsection

The Language-selection combobox does not work properly with touch and
the long list of language and country-variants is almost impossible to
use. (This is a revision of PR #279 -- as per the recommendation of
Gonzalo, no ellipsis or max character width is used since we have
ample room.)

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