Doc: fix app type notes for QDirModel and QFileSystemModel

"GUI application" is quite confusing, because in fact, these models
are part of QtWidgets and thus require QApplication instead of
QGuiApplication.

Change-Id: I35ccc8bfeb74c75a84b2ffbe1cab0c0839495980
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>
Reviewed-by: Topi Reiniö <topi.reinio@theqtcompany.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/widgets/dialogs/qfilesystemmodel.cpp

@@ -81,7 +81,7 @@ QT_BEGIN_NAMESPACE
     about the underlying files and directories related to items in the model.
     Directories can be created and removed using mkdir(), rmdir().
 
-    \note QFileSystemModel requires an instance of a GUI application.
+    \note QFileSystemModel requires an instance of \l QApplication.
 
     \section1 Example Usage
 

src/widgets/itemviews/qdirmodel.cpp

@@ -202,7 +202,7 @@ void QDirModelPrivate::invalidate()
     Directories can be created and removed using mkdir(), rmdir(), and the
     model will be automatically updated to take the changes into account.
 
-    \note QDirModel requires an instance of a GUI application.
+    \note QDirModel requires an instance of \l QApplication.
 
     \sa nameFilters(), setFilter(), filter(), QListView, QTreeView, QFileSystemModel,
     {Dir View Example}, {Model Classes}