Fix auto transition when changing room without exit_scene event (#462)

Added a new fourth button in room14 to demonstrate this.
Renamed BYPASS_LAST_SCENE to FORCE_LAST_SCENE_NULL to make it a bit clearer.
Made events "finished" and "interrupted" signals connections as ONESHOT (so they are disconnected once the signal was received).

Also removed some unused files, many others remain.
And fixed some bugs here and there.

game/rooms/room12/room12.tscn

@@ -78,7 +78,7 @@ position = Vector2( 340.052, 298.812 )
 
 [node name="r_door" type="Area2D" parent="Hotspots"]
 script = ExtResource( 8 )
-global_id = "r_door"
+global_id = "r12_r_exit"
 tooltip_name = "Right exit"
 target_scene = "res://game/rooms/room13/room13.tscn"
 switch_sound = "res://game/sfx/sounds/doorOpen_2.ogg"
@@ -94,3 +94,4 @@ script = ExtResource( 5 )
 position = Vector2( 542.824, 468.193 )
 script = ExtResource( 5 )
 global_id = "player_start"
+is_start_location = true