lvgl: add compatibility layer for both v7 and v8

Author: facchinm

libraries/Portenta_lvgl/examples/Portenta_lvgl/Portenta_lvgl.ino

@@ -7,6 +7,9 @@ void setup() {
 }
 
 void loop() {
+#if LVGL_VERSION_MAJOR > 7
+  lv_timer_handler();
+#else
   lv_task_handler();
-  delay(3);
+#endif
 }

libraries/Portenta_lvgl/examples/Portenta_lvgl/lv_demo_widgets.c

@@ -9,6 +9,8 @@
 #include "lvgl.h"
 #include "lv_demo_widgets.h"
 
+#if LVGL_VERSION_MAJOR == 7
+
 /*********************
  *      DEFINES
  *********************/
@@ -408,3 +410,5 @@ static void tab_changer_task_cb(lv_task_t * task)
 
     lv_tabview_set_tab_act(tv, act, LV_ANIM_ON);
 }
+
+#endif