MarkdownView: Merge into TextEditor

Make it a bit more compact, too.

Change-Id: I448164371c5a1d6969a292feba17caa2826f58c3
Reviewed-by: Tasuku Suzuki <tasuku.suzuki@signal-slot.co.jp>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/texteditor/texteditorplugin.cpp

@@ -10,6 +10,7 @@
 #include "highlighter.h"
 #include "icodestylepreferences.h"
 #include "linenumberfilter.h"
+#include "markdowneditor.h"
 #include "outlinefactory.h"
 #include "plaintexteditorfactory.h"
 #include "snippets/snippetprovider.h"
@@ -66,6 +67,7 @@ public:
     FindInOpenFiles findInOpenFilesFilter;
 
     PlainTextEditorFactory plainTextEditorFactory;
+    MarkdownEditorFactory markdownEditorFactory;
 };
 
 static TextEditorPlugin *m_instance = nullptr;