Re-implement the Vex content buffer as a view on the source buffer. [was #7834]
As first step towards TG-378, get rid of the most fragile and most inefficient part of the Linked Document Model: The (Vex) content buffer.
Somewhere inside the Structured Text framework, there's a framework to implement an editable view on specific parts of a text editor's buffer (here: the source view). This could be used to replace the Vex Content buffer as well as the whole translate-between-source-and-content-positions stuff.