feat(event) rework the prototype of lv_event_cb

It encapsulates all event related parameters into a single lv_event_t obejct.

examples/widgets/btn/lv_example_btn_1.c

@@ -1,13 +1,14 @@
 #include "../../lv_examples.h"
 #if LV_USE_BTN && LV_BUILD_EXAMPLES
 
-static void event_handler(lv_obj_t * obj, lv_event_t event)
+static void event_handler(lv_event_t * e)
 {
-    LV_UNUSED(obj);
-    if(event == LV_EVENT_CLICKED) {
+    lv_event_code_t code = lv_event_get_code(e);
+
+    if(code == LV_EVENT_CLICKED) {
         LV_LOG_USER("Clicked");
     }
-    else if(event == LV_EVENT_VALUE_CHANGED) {
+    else if(code == LV_EVENT_VALUE_CHANGED) {
         LV_LOG_USER("Toggled");
     }
 }