feat(event) rework the prototype of lv_event_cb

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

examples/widgets/dropdown/lv_example_dropdown_1.c

@@ -1,14 +1,14 @@
 #include "../../lv_examples.h"
 #if LV_USE_DROPDOWN && LV_BUILD_EXAMPLES
 
-static void event_handler(lv_obj_t * obj, lv_event_t event)
+static void event_handler(lv_event_t * e)
 {
-    if(event == LV_EVENT_VALUE_CHANGED) {
-#if LV_USE_LOG      /*To avoid warnings if LV_LOG_USER is empty*/
+    lv_event_code_t code = lv_event_get_code(e);
+    lv_obj_t * obj = lv_event_get_target(e);
+    if(code == LV_EVENT_VALUE_CHANGED) {
         char buf[32];
         lv_dropdown_get_selected_str(obj, buf, sizeof(buf));
         LV_LOG_USER("Option: %s", buf);
-#endif
     }
 }