Add macro enabling qobject_cast for template class.
Add macro enabling qobject_cast for template class.

d-tk/dtk,rdebroiz/dtk,rdebroiz/dtk,d-tk/dtk,rdebroiz/dtk,NicolasSchnitzler/dtk,NicolasSchnitzler/dtk,d-tk/dtk,rdebroiz/dtk,NicolasSchnitzler/dtk,d-tk/dtk,d-tk/dtk,NicolasSchnitzler/dtk,d-tk/dtk