[Core] Add visibility toggling to ViewProvider

With this extension of the API, view providers can indicate whether
document objects should be able to be toggled for visibility.  There is
both a C++ and Python interface, idiomatic for FreeCAD code.

src/Gui/ViewProvider.cpp

@@ -93,6 +93,7 @@ PROPERTY_SOURCE_ABSTRACT(Gui::ViewProvider, App::TransactionalObject)
 
 ViewProvider::ViewProvider()
     : overrideMode("As Is")
+    , toggleVisibilityMode(ToggleVisibilityMode::CanToggleVisibility)
 {
     setStatus(UpdateData, true);