Drive simulation iterations from Slicer

Use `SimulationMode::Backend` functionality introduced in https://github.com/Kitware/iMSTK/commit/55a69b687b78536d493751d50d3a561dbbf23cb4 to drive the simulation loop from Slicer (on a timer or something, we mostly just want to verify that this works)

_This issue was originally created by @sjh26 on August 26th 2019 as `https://gitlab.kitware.com/iMSTK/slicerimstk/-/issues/2`_

Start with the SimulationMode::Backend functionality introduced in commit 55a69b687b78536d493751d50d3a561dbbf23cb4 and inspect how Slicer can invoke simulation iterations. Verify the integration by driving the loop from Slicer, potentially using a timer, and confirm that the iterations run as expected.

Written by the indexing model from the issue text.