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, 22 insertions, 2 deletions

diff --git a/release/scripts/startup/bl_ui/__init__.py b/release/scripts/startup/bl_ui/__init__.py
index ef705f8fe37..25484e905c3 100644
--- a/release/scripts/startup/bl_ui/__init__.py
+++ b/release/scripts/startup/bl_ui/__init__.py
@@ -117,13 +117,15 @@ def register():
         for cls in mod.classes:
             register_class(cls)
 
-    # space_userprefs.py
     from bpy.props import (
         EnumProperty,
         StringProperty,
     )
-    from bpy.types import WindowManager
+    from bpy.types import (
+        WindowManager,
+    )
 
+    # space_userprefs.py
     def addon_filter_items(_self, _context):
         import addon_utils
 
@@ -234,3 +236,21 @@ class UI_UL_list(bpy.types.UIList):
 
 
 bpy.utils.register_class(UI_UL_list)
+
+
+class UI_MT_list_item_context_menu(bpy.types.Menu):
+    """
+    UI List item context menu definition. Scripts can append/prepend this to
+    add own operators to the context menu. They must check context though, so
+    their items only draw in a valid context and for the correct UI list.
+    """
+
+    bl_label = "List Item"
+    bl_idname = "UI_MT_list_item_context_menu"
+
+    def draw(self, context):
+        # Dummy function. This type is just for scripts to append their own
+        # context menu items.
+        pass
+
+bpy.utils.register_class(UI_MT_list_item_context_menu)