Python: Add new `annotation_pre` & `annotation_post` handlers

Annotation tool is used as a general mark tool for many add-ons. To be able to detect when an annotation is done is very handy to integrate the annotation tool in add-ons and other studio workflows.

The  new callback names are:  `annotation_pre`  and `annotation_post`

Both callbacks are exposed via the Python module `bpy.app.handlers`

Example use:

```
import bpy

def annotation_starts(gpd):
    print("Annotation starts")

def annotation_done(gpd):
    print("Annotation done")

bpy.app.handlers.annotation_pre.clear()
bpy.app.handlers.annotation_pre.append(annotation_starts)
bpy.app.handlers.annotation_post.clear()
bpy.app.handlers.annotation_post.append(annotation_done)
```

Note: The handlers are called for any annotation tool, including eraser.

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D14221

1 files changed, 2 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_callbacks.h b/source/blender/blenkernel/BKE_callbacks.h
index 66089d29c45..2cd28c4dfa5 100644
--- a/source/blender/blenkernel/BKE_callbacks.h
+++ b/source/blender/blenkernel/BKE_callbacks.h
@@ -95,6 +95,8 @@ typedef enum {
   BKE_CB_EVT_LOAD_FACTORY_USERDEF_POST,
   BKE_CB_EVT_LOAD_FACTORY_STARTUP_POST,
   BKE_CB_EVT_XR_SESSION_START_PRE,
+  BKE_CB_EVT_ANNOTATION_PRE,
+  BKE_CB_EVT_ANNOTATION_POST,
   BKE_CB_EVT_TOT,
 } eCbEvent;