[Feature Request] Objects UI has a collapsed group containing non-explictly shown objects

```python
import cadquery as cq

a=cq.xxxxxx .....
b=cq.xxxxxx .....
c=cq.xxxxxx .....
```
If we don't use `show_object()`, then `a`,`b`,`c` are shown in CQ-editor's Object UI.

If we use
```python
show_object(c)
```
then `a`,`b` are ignored completely in CQ-editor.

Please add a group to Objects UI. It is collapsed by default. It contains `a`, `b` and those non-explictly shown objects which are set to invisible by default. User can manually enable object in that group to make a object visible.