Minor updates
Lumps a handful of small smtk updates together.
- Changes qtFileItem.cxx to only create a file filter when one is specified in the definition. Removes the need to add one (and get the syntax right) for when using in directory mode.
- Add pybind11 methods for
smtk::operation::Manager
registered()
andunregisterOperation()
methods, so that we can use them, e.g., in the python shell. - Update
smtk::AttributeBuilder
to allow multiple instanced atts of the same type.