Feature/#137 default engine setup

Default engine setup sets up engine with (at the moment)

• a rendering system
• an input system

Due to the way we define key callbacks in the input system now, no default key bindings are set (there is only one callback for all keys, so the application has to set all key callbacks and cannot just add one).

The scene is not owned by the engine, which is why it still has to be created on application side. The window is now managed by the engine. The default setup at the moment looks like this:

auto scene = std::make_unique<phx::Scene>();
auto engine = phx::Setup::CreateDefaultEngine(scene.get());
engine->Run();

@ademiralp @mbellgardt

Closes #137