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.h

@@ -58,7 +58,7 @@ public:
     virtual ~BaseTextDocument();
 
     // ITextEditorDocument
-    QString contents() const;
+    QString plainText() const;
     QString textAt(int pos, int length) const;
     QChar characterAt(int pos) const;