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/texteditor/texteditor.h

@@ -556,6 +556,7 @@ public:
     void setContextHelpItem(const Core::HelpItem &item);
 
     static TextEditorWidget *currentTextEditorWidget();
+    static TextEditorWidget *fromEditor(const Core::IEditor *editor);
 
 protected:
     /*!
@@ -641,7 +642,8 @@ public:
 
     using EditorCreator = std::function<BaseTextEditor *()>;
     using DocumentCreator = std::function<TextDocument *()>;
-    using EditorWidgetCreator = std::function<TextEditorWidget *()>;
+    // editor widget must be castable (qobject_cast or Aggregate::query) to TextEditorWidget
+    using EditorWidgetCreator = std::function<QWidget *()>;
     using SyntaxHighLighterCreator = std::function<SyntaxHighlighter *()>;
     using IndenterCreator = std::function<Indenter *(QTextDocument *)>;
     using AutoCompleterCreator = std::function<AutoCompleter *()>;