refactor: replace filename hyphens with underscores

Aligns *.h and *.c to underscore naming convention.

These were kept (with warnings) for backwards compatibility with external boards/shields:
- kscan-mock.h
- matrix-transform.h
They should be removed in the future.

PR: #523

app/include/zmk/events/activity_state_changed.h

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <zephyr.h>
+#include <zmk/event_manager.h>
+#include <zmk/activity.h>
+
+struct activity_state_changed {
+    struct zmk_event_header header;
+    enum zmk_activity_state state;
+};
+
+ZMK_EVENT_DECLARE(activity_state_changed);
+
+static inline struct activity_state_changed *
+create_activity_state_changed(enum zmk_activity_state state) {
+    struct activity_state_changed *ev = new_activity_state_changed();
+    ev->state = state;
+
+    return ev;
+}