TextEditors: Separate away methods that actually work on a document

Introduces ITextEditorDocument. This is part of a more general "use
documents instead of editors whereever possible". It will allow to move
to e.g. ITextEditor::openedTextDocumentContents() instead of
ITextEditor::openedTextEditorsContents().

Change-Id: I5ebceaa257a0d2c3e8ac4ac51b9b08b6faa42487
Reviewed-by: David Schulz <david.schulz@digia.com>