Add a helper class to wait for event handlers to complete
Add a helper class to wait for event handlers to complete

RafaelPalomar/girder,jbeezley/girder,manthey/girder,RafaelPalomar/girder,girder/girder,data-exp-lab/girder,manthey/girder,Kitware/girder,data-exp-lab/girder,data-exp-lab/girder,kotfic/girder,data-exp-lab/girder,kotfic/girder,Kitware/girder,jbeezley/girder,kotfic/girder,kotfic/girder,data-exp-lab/girder,jbeezley/girder,RafaelPalomar/girder,jbeezley/girder,Xarthisius/girder,Xarthisius/girder,Kitware/girder,Xarthisius/girder,manthey/girder,RafaelPalomar/girder,girder/girder,manthey/girder,girder/girder,Kitware/girder,RafaelPalomar/girder,girder/girder,kotfic/girder,Xarthisius/girder,Xarthisius/girder