This commit makes GTK_ALIGN_START/_END pay attention to
the text direction when used in horizontal context.

This is how similar parameters in GtkMisc and GtkAlignment work,
and is generally expected of GTK+ positioning parameters. And this
is new GTK+ 3 api, so it is basically still unused at this point.

If explicit right/left turn out to be needed at some point, we
can expand the enumeration with new values.