Setup preliminary Game menus and Scene transitions

[Cataclysmanic]

Basics: Splash Screen, main menu, end-screen. Maybe transitions. Incorporate audio and animation if needed.

Switch to scenes entirely, or have Main.tscn instantiate and free levels as needed.

[plexsoup]

I modified the scene transitions, so they no longer rely on get_tree().change_scene(). Now, they use the change_scene() method on Main.gd. To call it, use: Global.world_controller.change_scene(pathToScene), or Global.world_controller.change_scene_to(packedScene)

This will let us keep Main.tscn active in memory for things like loading and saving game files.

[plexsoup]

added a basic rain shader to the main menu