Refactor global bindings, implement mod-tap.

* Use extra comptible = "zmk,behavior-global" to add
   behaviors to global bindings for event notification.
* Implement mod-tap, as a keymap binding and global one
  to skip tap if other keycode pressed while held.

app/include/zmk/keymap.h

@@ -1,13 +1,5 @@
 #pragma once
 
-#include <devicetree.h>
-#include <usb/usb_device.h>
-#include <usb/class/usb_hid.h>
-#include <dt-bindings/zmk/keys.h>
-
-#include <zmk/matrix.h>
-// #include <zmk/keys.h>
-
 bool zmk_keymap_layer_activate(u8_t layer);
 bool zmk_keymap_layer_deactivate(u8_t layer);