Move displayName from IEditor to IDocument

The display name is not editor instance specific, but belongs to the
document.

Change-Id: I3c936f04a86e10e6ca30063d85036d85b4b5880e
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/plugins/texteditor/plaintexteditor.cpp

@@ -69,7 +69,6 @@ PlainTextEditorWidget::PlainTextEditorWidget(QWidget *parent)
     setIndenter(new NormalIndenter); // Currently only "normal" indentation is supported.
 
     setMimeType(QLatin1String(TextEditor::Constants::C_TEXTEDITOR_MIMETYPE_TEXT));
-    setDisplayName(tr(Core::Constants::K_DEFAULT_TEXT_EDITOR_DISPLAY_NAME));
 
     m_commentDefinition.clearCommentStyles();