- the new variable makes it easier to implement language buttons
- a short howto on language buttons is also provided
- the dictionary mapping template vars are now OrderedDict
for consistent-looking language buttons
this commit removes the need to make a dummy translation for
the language in which the templates are written.
This only affects themes using jinja2.ext.i18n.
The I18N_THEMES_LANG is introduced to address this issue.
Also expanded the docs for making gettext .po files with babel.