o.c.scan: Fix simulation config issues

Simulation config is required, but it can be 'empty', meaning: Use the
beamline config file, don't define any slew rates.

When simulation config path is set to empty, it will now default to the
beamline config path.

beamline.xsd now includes the slew_rate.
o.c.archive.engine: Remove 'SHOW_PROGRESS_ON_STARTUP'

IDE had added that
Merge pull request #224 from ControlSystemStudio/bug222_scan_simu_config

Bug222 scan simu config: resolves #222