Synchronize HID keyboard state on first event

When an AOA HID keyboard is registered, CAPSLOCK and NUMLOCK are both
disabled, regardless of the state of the computer keyboard.

To synchronize the state, on first key event, inject CAPSLOCK and/or
NUMLOCK if necessary.

app/src/hid_keyboard.h

@@ -31,6 +31,8 @@ struct sc_hid_keyboard {
 
     struct sc_aoa *aoa;
     bool keys[SC_HID_KEYBOARD_KEYS];
+
+    bool mod_lock_synchronized;
 };
 
 bool