feature(keymaps): Keymap layer state event, highest layer API

* Fire an event w/ layer number, state, and timestamp
  when a layer is activated/deactivated.
* Add `zmk_keymap_highest_layer_active()` query.

Co-authored-by: KemoNine <mcrosson@kemonine.info>

app/include/zmk/events/layer-state-changed.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <zephyr.h>
+#include <zmk/event-manager.h>
+
+struct layer_state_changed {
+    struct zmk_event_header header;
+    u8_t layer;
+    bool state;
+    s64_t timestamp;
+};
+
+ZMK_EVENT_DECLARE(layer_state_changed);
+
+static inline struct layer_state_changed *create_layer_state_changed(u8_t layer, bool state) {
+    struct layer_state_changed *ev = new_layer_state_changed();
+    ev->layer = layer;
+    ev->state = state;
+    ev->timestamp = k_uptime_get();
+
+    return ev;
+}