With this patch, you can now assign the same GTextDocument to multiple
GTextEditor widgets via GTextEditor::set_document().
The editors have independent cursors and selection, but all changes
are shared, and immediately propagate to all editors.
This is very unoptimized and will do lots of unnecessary invalidation,
especially line re-wrapping and repainting over and over again.
This patch decouples GTextDocument and GTextDocumentLine from the line
wrapping functionality of GTextEditor.
This should basically make it possible to have multiple GTextEditors
editing the same GTextDocument. Of course, that will require a bit more
work since there's no paint invalidation yet.
The idea here is to decouple the document from the editor widget so you
could have multiple editors being views onto the same document.
This doesn't work yet, since the document and editor are coupled in
various ways still (including a per-line back-pointer to the editor.)
