Refactor app loading and window management (#609)

Modules/app-module/include/app/scheduler.h

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: Apache-2.0
+#pragma once
+
+#include <app/instance.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @return the app_instance_id of whichever app instance's task is calling this (every app
+ * instance's task stashes it in its own thread-local storage when it starts), or 0 if called
+ * from a task that isn't a running app instance. An app's own main() typically calls this once,
+ * near the top, to learn its own instance id - see e.g. app_event_subscribe()/
+ * window_manager_create(), both of which need it.
+ */
+AppInstanceId app_scheduler_current_app_id(void);
+
+#ifdef __cplusplus
+}
+#endif