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>

src/plugins/texteditor/basetextdocument.cpp

@@ -92,7 +92,7 @@ BaseTextDocument::~BaseTextDocument()
     delete d;
 }
 
-QString BaseTextDocument::contents() const
+QString BaseTextDocument::plainText() const
 {
     return document()->toPlainText();
 }