Rename QString ITextEditorDocument::contents --> plainText

To differentiate it from the byte array based IDocument::setContents

Change-Id: Icc9600732c2742fb79254a0697870bacce3a59ee
Reviewed-by: David Schulz <david.schulz@digia.com>
Move markableInterface() to ITextEditorDocument

And remove the indirection over BaseTextEditorWidget.

Change-Id: I2c570edf46b9ca72a11704ce27d8ae3f2218dcf0
Reviewed-by: David Schulz <david.schulz@digia.com>