#6 Implement vehicle loading box in the main window

[Gordon Tisher]

Implement the vehicle loading box in the main window (the simulation tab, actually).

Allow the user to choose a vehicle (defined in the loaded packages), and its position relative to the planet selected in the sim_view_box.

Then modify the simulation config that gets written out (in handle_run_simulation() in simulation_tab.cpp), so that when the simulation loads, the view is in the vehicle cockpit.

There's an empty file (sim_vehicle_box.hpp/cpp) that is intended for this.