make `show_messages` user configurable

[morganrallen]

The variable show_messages hard-coded to True, this causes a bunch of popup windows during loading, most of which aren't that useful once you're familiar with StepUp. Additionally I'm experimenting with automating kicadStepUp and these popups are becoming increasingly problematic as they stall the process until dismissed.
It would be great if this variable could be moved into the User Preferences.