Fix T100423: Addon's custom context menu entries get overridden by other addons

This introduces a new `UI_MT_button_context_menu` class which is
registered at startup. Addons can append/prepend draw functions to this
class, in order to add their custom context menu entries.

The new class replaces the old `WM_MT_button_context` class, thus
requiring a small change in addons using this feature. This is done
because addons were previously required to register the class
themselves, which caused addons to override each other's context menu
entries.

Now the class registration is handled by Blender, and addons need only
append their draw functions. The new class name ensures that addons
using the old method don't override menu entries made using the new
class.

Menu entries added with the legacy `WM_MT_button_context` class are
still drawn for backwards compatibility, but this class must not be used
going forward, as any addon using it still runs the risk of having its
menu entries overridden, and support for the legacy class is subject to
removal in a future version.

Reviewed By: campbellbarton

Maniphest Tasks: T100423

Differential Revision: https://developer.blender.org/D15702

release/scripts/addons

@@ -1 +1 @@
-Subproject commit 7a8502871c34db0343cc7de52d6b49b15a84238a
+Subproject commit 32baafe44dc56edd1baf6e5a16b4439ded8238f2

release/scripts/addons_contrib

@@ -1 +1 @@
-Subproject commit 95107484d076bc965239942e857c83433bfa86d7
+Subproject commit 42da56aa73726710107031787af5eea186797984

release/scripts/startup/bl_ui/__init__.py

@@ -239,3 +239,23 @@ class UI_MT_list_item_context_menu(bpy.types.Menu):
 
 
 bpy.utils.register_class(UI_MT_list_item_context_menu)
+
+
+class UI_MT_button_context_menu(bpy.types.Menu):
+    """
+    UI button 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 buttons.
+    """
+
+    bl_label = "List Item"
+    bl_idname = "UI_MT_button_context_menu"
+
+    def draw(self, context):
+        # Draw menu entries created with the legacy `WM_MT_button_context` class.
+        # This is deprecated, and support will be removed in a future release.
+        if hasattr(bpy.types, "WM_MT_button_context"):
+            self.layout.menu_contents("WM_MT_button_context")
+
+
+bpy.utils.register_class(UI_MT_button_context_menu)