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/clangcodemodel/test/clangautomationutils.cpp

@@ -106,7 +106,7 @@ TextEditor::ProposalModelPtr completionResults(TextEditor::BaseTextEditor *textE
 {
     using namespace TextEditor;
 
-    auto textEditorWidget = qobject_cast<TextEditorWidget *>(textEditor->widget());
+    auto textEditorWidget = TextEditorWidget::fromEditor(textEditor);
     QTC_ASSERT(textEditorWidget, return TextEditor::ProposalModelPtr());
     AssistInterface *assistInterface = textEditorWidget->createAssistInterface(
                 TextEditor::Completion, TextEditor::ExplicitlyInvoked);