add steam overlay, achievements and stats managers

core/steam_manager/steam_manager.gd

@@ -0,0 +1,27 @@
+extends Node
+
+const APP_ID: int = 4809260		#Train Goes Choo Choo
+
+var is_on_steam: bool = false
+
+func _init() -> void:
+	OS.set_environment("SteamAppId", str(APP_ID))
+	OS.set_environment("SteamGameId", str(APP_ID))
+
+func _ready() -> void:
+	process_mode = Node.PROCESS_MODE_ALWAYS
+
+	var init := Steam.steamInitEx()
+	if init['status'] != Steam.STEAM_API_INIT_RESULT_OK:
+		push_error("Steam not initialized: %s" % init['verbal'])
+		return
+
+	is_on_steam = true
+	Steam.overlay_toggled.connect(_on_overlay_toggled)
+
+func _process(_delta: float) -> void:
+	if is_on_steam:
+		Steam.run_callbacks()   # every frame, always
+
+func _on_overlay_toggled(active: bool, _user_initiated: bool, _app_id: int) -> void:
+	get_tree().paused = active