Add TextDocument with full size editor

A new document object App::TextDocument. It has a property Text which
holds the text of the document as a string. This commit also contains a
full size editor based on QPlainTextEdit. It can only be used by the
TextDocument and has a read only mode invoked for read only documents
(ReadOnly property set to true). The editor is invoked by a double
click on the TextDocument.

src/App/Application.cpp

@@ -106,6 +106,7 @@
 #include "OriginFeature.h"
 #include "Origin.h"
 #include "MaterialObject.h"
+#include "TextDocument.h"
 #include "Expression.h"
 #include "Transactions.h"
 #include <App/MaterialPy.h>
@@ -1301,6 +1302,7 @@ void Application::initTypes(void)
     App ::MeasureDistance           ::init();
     App ::MaterialObject            ::init();
     App ::MaterialObjectPython      ::init();
+    App ::TextDocument              ::init();
     App ::Placement                 ::init();
     App ::OriginFeature             ::init();
     App ::Plane                     ::init();