Start: enable multiple custom folder paths

* Update tooltip to indicate support for multiple custom folder paths
* Remove migration step that worked around lack of multiple folder support
* Remove obsolete migration method
* Address linter warnings: build/include_what_you_use
* Add QStringList as requested on https://github.com/FreeCAD/FreeCAD/pull/19948/files#r1982267583

src/Mod/Start/App/CustomFolderModel.h

@@ -25,7 +25,7 @@
 #define FREECAD_START_CUSTOMFOLDERMODEL_H
 
 #include <QAbstractListModel>
-#include <QDir>
+#include <QString>
 #include <Base/Parameter.h>
 
 #include "DisplayedFilesModel.h"
@@ -45,7 +45,7 @@ public:
     void loadCustomFolder();
 
 private:
-    QDir _customFolderDirectory;
+    QString _customFolderPathSpec;
     bool _showOnlyFCStd;  // Show only FreeCAD files
 };