Add the ability to set the text direction

Add the ability to set the text direction

The direction of the text depends on the locale, and it is the
basic setting needed to enable internationalization of user
interfaces.

This commit allows setting the direction of the PangoContext instance
used by Clutter by using the CLUTTER_TEXT_DIRECTION environment
variable, or by passing the --clutter-text-direction command line
argument. Valid values are:

        ltr     - for left-to-right locales
        rtl     - for right-to-left locales

The default is LTR.

Ideally, this should be a value set by the localization teams on the
PO file, but this step requires some build system surgery to allow
the translation of the Clutter strings.