UI: Support UI list tooltips, defined via Python scripts

Makes it possible to create tooltips for UI list rows, which can be
filled in .py scripts, similar to how they can extend other menus. This
is used by the (to be committed) Pose Library add-on to display pose
operations (selecting bones of a pose, blending a pose, etc).

It's important that the Python scripts check if the UI list is the
correct one by checking the list ID.
For this to work, a new `bpy.context.ui_list` can be checked. For
example, the Pose Library add-on does the following check:
```
def is_pose_asset_view() -> bool:
  # Important: Must check context first, or the menu is added for every kind of list.
  list = getattr(context, "ui_list", None)
  if not list or list.bl_idname != "UI_UL_asset_view" or list.list_id != "pose_assets":
    return False
  if not context.asset_handle:
    return False
  return True
```

1 files changed, 11 insertions, 0 deletions

diff --git a/source/blender/editors/screen/screen_context.c b/source/blender/editors/screen/screen_context.c
index ece6ba986f3..3ce2f326dca 100644
--- a/source/blender/editors/screen/screen_context.c
+++ b/source/blender/editors/screen/screen_context.c
@@ -112,6 +112,7 @@ const char *screen_context_dir[] = {
     "selected_editable_fcurves",
     "active_editable_fcurve",
     "selected_editable_keyframes",
+    "ui_list",
     "asset_library",
     NULL,
 };
@@ -1034,6 +1035,15 @@ static eContextResult screen_ctx_asset_library(const bContext *C, bContextDataRe
   return CTX_RESULT_OK;
 }
 
+static eContextResult screen_ctx_ui_list(const bContext *C, bContextDataResult *result)
+{
+  wmWindow *win = CTX_wm_window(C);
+  ARegion *region = CTX_wm_region(C);
+  uiList *list = UI_list_find_mouse_over(region, win->eventstate);
+  CTX_data_pointer_set(result, NULL, &RNA_UIList, list);
+  return CTX_RESULT_OK;
+}
+
 /* Registry of context callback functions. */
 
 typedef eContextResult (*context_callback)(const bContext *C, bContextDataResult *result);
@@ -1109,6 +1119,7 @@ static void ensure_ed_screen_context_functions(void)
   register_context_function("active_editable_fcurve", screen_ctx_active_editable_fcurve);
   register_context_function("selected_editable_keyframes", screen_ctx_selected_editable_keyframes);
   register_context_function("asset_library", screen_ctx_asset_library);
+  register_context_function("ui_list", screen_ctx_ui_list);
 }
 
 /* Entry point for the screen context. */