fix(gui): forward optional visibility arg in Workbench.appendToolbar()

The Python wrapper in FreeCADGuiInit.py only accepted (name, cmds) and
did not forward the optional visibility parameter to the underlying C++
PythonWorkbench::appendToolbar which accepts 'sO|s'. This caused a
'takes 3 positional arguments but 4 were given' error when workbenches
passed a visibility string like 'Unavailable'.

src/Gui/FreeCADGuiInit.py

@@ -113,8 +113,11 @@ class Workbench:
     def ContextMenu(self, recipient):
         pass
 
-    def appendToolbar(self, name, cmds):
-        self.__Workbench__.appendToolbar(name, cmds)
+    def appendToolbar(self, name, cmds, visibility=None):
+        if visibility is not None:
+            self.__Workbench__.appendToolbar(name, cmds, visibility)
+        else:
+            self.__Workbench__.appendToolbar(name, cmds)
 
     def removeToolbar(self, name):
         self.__Workbench__.removeToolbar(name)