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/debugger/debuggerplugin.cpp

@@ -1835,7 +1835,7 @@ void DebuggerPluginPrivate::runScheduled()
 
 void DebuggerPluginPrivate::editorOpened(IEditor *editor)
 {
-    if (auto widget = qobject_cast<TextEditorWidget *>(editor->widget())) {
+    if (auto widget = TextEditorWidget::fromEditor(editor)) {
         connect(widget, &TextEditorWidget::markRequested,
                 this, &DebuggerPluginPrivate::requestMark);