lv_anim_set_custom_exec_cb() function

Similar to `lv_anim_set_exec_cb` but `lv_anim_custom_exec_cb_t` receives `lv_anim_t * ` as its first parameter instead of `void *`. This function might be used when LVGL is bound to other languages because it's more consistent to have `lv_anim_t *` as first parameter. The variable to animate can be stored in the animation's `user_data`