Manage several scenarios

The designer should be able to open several scenarios at the same time, in various tabs.

Each tab should have its own control system (play, stop, save...) and each scenario can be run in parallel.

• Create an 'Untitled' tab with empty scenario at startup, if no scenario is already open
• Open one or mutliple scenarios with an explorer dialog and open each selected in a separate tab
• Save a scenario (if never saved, open a dialog to choose name and location).