bug 251955: Basic support for processing instructions

Added read/write support for processing insctructions, so they don't get
lost when editing a document with VEX.
To show the PI's in the layout will be a seperate step.

Change-Id: I20a1fd4d8ff8199038b6d8ddbf1d268aff1be57b
Signed-off-by: Carsten Hiesserich <carsten.hie@gmail.com>
22 files changed