feat: add new operator about convert curve to mesh with group infos.

- add a new operator which can converte selected object to mesh, and if object is curve and has bevel object, it will copy the virtools group info of bevel object at the same time.
- add a hint in virtools group panel to tell user that the virtools group of non-mesh object will not be saved.

bbp_ng/PROP_virtools_group.py

@@ -377,7 +377,13 @@ class BBP_PT_virtools_groups(bpy.types.Panel):
     
     def draw(self, context):
         layout = self.layout
-        target = bpy.context.active_object
+        target = typing.cast(bpy.types.Object, context.active_object)
+
+        # notify on non-mesh object
+        if target.type != 'MESH':
+            layout.label(text = 'Virtools Group is invalid on non-mesh object!', icon = 'ERROR')
+
+        # draw main body
         
         row = layout.row()
         row.template_list(