add basic XInclude support

With this commit, Vex supports documents that use XInclude's. The
functionality is still very basic, but at least the XInclude tags are
displayed and preserved when editing a document with Vex.

Change-Id: I5d67b23fcebdeba95cf0be097173cd36cc9b4bf4
Signed-off-by: Carsten Hiesserich <carsten.hie@gmail.com>
33 files changed