fix(WidgetManager): handle touch start events
fix(WidgetManager): handle touch start events

Touch is currently translated as a LeftButtonPress event.

In supporting touch, we need to "pause" the LeftButtonPress event until
we have updated the widget manager's selection. Otherwise, it's
impossible to select a widget's handle via the color buffer.

Kitware/vtk-js,Kitware/vtk-js,Kitware/vtk-js,Kitware/vtk-js