This breaks the view_item.h -> inspectable.h -> property.h chain that
pulled wx/propgrid/property.h into 150-200 TUs. The lean inspectable.h
now includes only wx/any.h, wx/string.h, wx/variant.h.
Headers using DECLARE_ENUM_TO_WXANY (board_item.h, sch_item.h) retain
their property.h include since the macro requires it.