Add MRML node combobox as dose engine plugin beam parameter type

[cpinter]

Add a new beam parameter type to select a MRML node (such as an ROI), similarly to the existing parameters https://github.com/SlicerRt/SlicerRT/blob/master/ExternalBeamPlanning/DoseEngines/qSlicerAbstractDoseEngine.h#L113-L179

[cpinter]

Will need to update the orthovoltage plugin to use this for ROI selection.