TextEditor: Use more direct approach for fallback IEditor construction

There are still a few places where the EditorWidget is the primary
object, or several of them per "visible" IEditor (e.g. Diff).
Provide a means to streamline setup there, too.

Change-Id: I14cfbd68e555ebc539e707032a0e5bef563e0a36
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/diffeditor/unifieddiffeditorwidget.cpp

@@ -81,6 +81,8 @@ UnifiedDiffEditorWidget::UnifiedDiffEditorWidget(QWidget *parent)
     , m_leftLineNumberDigits(1)
     , m_rightLineNumberDigits(1)
 {
+    setupFallBackEditor(Id());
+
     DisplaySettings settings = displaySettings();
     settings.m_textWrapping = false;
     settings.m_displayLineNumbers = true;
@@ -376,11 +378,6 @@ void UnifiedDiffEditorWidget::patch(int diffFileIndex, int chunkIndex, bool reve
         m_controller->requestReload();
 }
 
-TextEditor::BaseTextEditor *UnifiedDiffEditorWidget::createEditor()
-{
-    return new BaseTextEditor;
-}
-
 void UnifiedDiffEditorWidget::clear(const QString &message)
 {
     m_leftLineNumberDigits = 1;