improve portals and scene transitions; tidy up old code #5

SceneTriggers and EntranceMarkers should now be configured through the 'connected_scene' and 'valid_prev_levels' properties in the inspector. Using @export_file lets us choose the specific level scenes, and Godot should keep track of them if we rename files etc.
I have a feeling we should link SceneTriggers to specific EntranceMarkers and vice versa (perhaps even wrapping them as Portals?), but this does the job for now :)

This adds a new SceneTransition scene containing a ColorRect and AnimationPlayer which can fade in or out. It's use in SceneManager to fade in/out when entering a door. Also had to allow disabling player input to avoid moving during the fade.

If doors in two levels were in similar positions, the player would spawn and trigger the door before it was repositioned. This commit updates SceneManager to only allow one scene transition at a time. There's probably a proper way to turn off physics or something but this seems to work.