Make TextEditor setup more flexible

So far it was only possible to combine TextEditorFactory, BaseTextEditor
and TextEditorWidget directly.
That TextEditorWidget is also directly a QPlainTextEdit made it
impossible to "decorate" the text editor widget with something else
without a lot of effort.

Make it possible to create a text editor factory that returns an
arbitrary widget, as long as it can be "cast" to a TextEditorWidget with
either qobject_cast or Aggregation::query. That way the TextEditorWidget
instance can be attached to the editor widget via Aggregation.

Adapt other code that accesses TextEditorWidget from editors
accordingly. Introduce a common method how to do that.

Change-Id: I72b8721f3a8a8d8281c39af75253e9c80cbe1250
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cpptools/cppcompletion_test.cpp

@@ -81,7 +81,7 @@ public:
         m_editor = EditorManager::openEditor(fileName);
         QVERIFY(m_editor);
         closeEditorAtEndOfTestCase(m_editor);
-        m_editorWidget = qobject_cast<TextEditorWidget *>(m_editor->widget());
+        m_editorWidget = TextEditorWidget::fromEditor(m_editor);
         QVERIFY(m_editorWidget);
 
         m_textDocument = m_editorWidget->document();