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/core/slider.md

@@ -36,8 +36,22 @@ In the latter case the knob moves to the point clicked and slider value changes
 ## Events
 - `LV_EVENT_VALUE_CHANGED` Sent while the slider is being dragged or changed with keys. 
 The event is sent continuously while the slider is dragged and once when released. Use `lv_slider_is_dragged` to detemine whether the Slider is still being dragged or has just been released.
+- `LV_EVENT_DRAW_PART_BEGIN` and `LV_EVENT_DRAW_PART_END` are sent for the following parts. 
+    - `LV_SLIDER_DRAW_PART_KNOB` The main (right) knob of the slider
+        - `part`: `LV_PART_KNOB` 
+        - `draw_area`: area of the indicator
+        - `rect_dsc`
+        - `id`: 0
+    - `LV_SLIDER_DRAW_PART_KNOB`  The left knob of the slider
+        - `part`: `LV_PART_KNOB` 
+        - `draw_area`: area of the indicator
+        - `rect_dsc`
+        - `id`: 1
+
+See the events of the [Bar](/widgets/core/bar) too.
 
 Learn more about [Events](/overview/event).
+
 ## Keys
 - `LV_KEY_UP/RIGHT` Increment the slider's value by 1
 - `LV_KEY_DOWN/LEFT` Decrement the slider's value by 1