lv_lock() function

Lock LVGL's general mutex. LVGL is not thread safe, so a mutex is used to avoid executing multiple LVGL functions at the same time from different threads. It shall be called when calling LVGL functions from threads different than lv_timer_handler's thread. It doesn't need to be called in LVGL events because they are called from lv_timer_handler(). It is called internally in lv_timer_handler().