CMake: Restructure main file, move example to its own directory

example/src/game.cpp

@@ -0,0 +1,15 @@
+#include "inferno.h"
+#include "inferno/entrypoint.h"
+
+class Game : public Inferno::Application
+{
+public:
+	Game() : Application({}) {}
+	~Game() {}
+};
+
+Inferno::Application& Inferno::createApplication()
+{
+	Game::initialize();
+	return Game::the();
+}