Pose Library: add conversion button to old poselib UI

Add the "Convert Old Pose Library" operator to the old pose library (in
the Armature properties editor). This makes it more discoverable; before
it only was available in the Action editor.

release/scripts/startup/bl_ui/properties_data_armature.py

@@ -220,6 +220,7 @@ class DATA_PT_pose_library(ArmatureButtonsPanel, Panel):
                 ).pose_index = poselib.pose_markers.active_index
 
             col.operator("poselib.action_sanitize", icon='HELP', text="")  # XXX: put in menu?
+            col.operator("poselib.convert_old_poselib", icon='ASSET_MANAGER', text="")
 
             if pose_marker_active is not None:
                 col.operator("poselib.pose_move", icon='TRIA_UP', text="").direction = 'UP'