QtDesignStudio: Split "New Files" from "New Project"

Just having the "New Project" dialog there is confusing to many users
and they do not know how to add new files.
Upstream in master we can remove the condition and split the dialog
also for Qt Creator.

Task-number: QDS-5494
Change-Id: I9a21f27fad932759fcfc412ecfac7f3f6d2bcfee
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Samuel Ghinet <samuel.ghinet@qt.io>

src/plugins/coreplugin/coreconstants.h

@@ -87,6 +87,7 @@ const char ZOOM_OUT[]              = "QtCreator.ZoomOut";
 const char ZOOM_RESET[]            = "QtCreator.ZoomReset";
 
 const char NEW[]                   = "QtCreator.New";
+const char NEW_FILE[]              = "QtCreator.NewFile";
 const char OPEN[]                  = "QtCreator.Open";
 const char OPEN_WITH[]             = "QtCreator.OpenWith";
 const char REVERTTOSAVED[]         = "QtCreator.RevertToSaved";