feat(event, widgets) improve the paramter of LV_EVENT_DRAW_PART_BEGIN/END

Add lv_<widget>_draw_part_type_t to widgets to precisly describe the hooked drawings.
Also add class_p element to lv_obj_draw_part_dsc_t to show what widgets lv_<widget>_draw_part_type_t needs to be used.

Related to: https://forum.lvgl.io/t/how-to-add-minor-division-lines-to-a-chart/5366/

docs/widgets/obj.md

@@ -149,6 +149,19 @@ By default, the objects can be clicked only on their coordinates, however, this
 
 ## Events
 - `LV_EVENT_VALUE_CHANGED` when the `LV_OBJ_FLAG_CHECKABLE` flag is enabled and the object clicked (on transition to/from the checked state)
+- `LV_EVENT_DRAW_PART_BEGIN` and `LV_EVENT_DRAW_PART_END` is sent for the following types:
+    - `LV_OBJ_DRAW_PART_RECTANGLE` The main rectangle
+       - `part`: `LV_PART_MAIN`
+       - `rect_dsc`
+       - `draw_area`: the area of the rectangle
+    - `LV_OBJ_DRAW_PART_BORDER_POST`  The border if the `border_post` style property is `true`
+       - `part`: `LV_PART_MAIN`
+       - `rect_dsc`
+       - `draw_area`: the area of the rectangle
+    - `LV_OBJ_DRAW_PART_SCROLLBAR` the scrollbars
+       - `part`: `LV_PART_SCROLLBAR`
+       - `rect_dsc`
+       - `draw_area`: the area of the rectangle
 
 Learn more about [Events](/overview/event).