Direct widget updates (e.g. after load) should be disabled in HEADLESS mode

[xparq]

Follow-up of #271

E.g., for each test run now:

- Warning: widget "Gravity mode" not found!
- WARNING: Widget "Gravity mode" not found! (WidgetPtr #000000880338E670 has been set to NULL.)
- ERROR: dereferencing NULL widget pointer!
- Warning: widget "Friction" not found!
- WARNING: Widget "Friction" not found! (WidgetPtr #000000880338E670 has been set to NULL.)
- ERROR: dereferencing NULL widget pointer!

due to

void OONApp::_on_snapshot_loaded()
{
    //
    // Sync the UI to reflect the new model state...
    //
    using namespace sfw;

    // Grav. mode:
    gui.set<GravityModeSelector>("Gravity mode", world().gravity_mode);

    // Drag:
    gui.set<Slider>("Friction", world().friction);

    ...
}

It's still safe, though, thanks to SFW's widget access API, but it's way too sloppy and noisy...

• [x] I guess a condition right in SFW to support this could be enough -- in fact just deactivating the entire UI should already do that! -> Well, not so simple: xparq/sfw2#464...
  • The "straightforward" conditions there could only remove two warnings (Warning: widget "..." not found! in GUI::recall) -- which could be better disabled by #ifdef DEBUG anyway, actually...
• [x] So, ended up just directly checking for it in _on_snapshot_loaded... :-/