Object tree selection handling for non AIS_Shape objects

(Low priority issue)

Thanks for #317

A little bit more fine tuning is still required at
https://github.com/CadQuery/CQ-editor/blob/0d091ba3d20114b4815f796ec48ab33f4ebbeb41/cq_editor/widgets/object_tree.py#L376

The statement works only for items of type AIS_Shape. To reproduce

1) Show a scene with at least one AIS_Shape and one other AIS_InteractiveObject
2) Click on the shape that is of type AIS_Shape
3) Following output appears in console

```
ERROR: CQ-Editor: Uncaught exception occurred
Traceback (most recent call last):
File "miniforge3/lib/python3.9/site-packages/cq_editor/widgets/object_tree.py", line 376, in handleGraphicalSelection
if item.ais.Shape().IsEqual(shape):
AttributeError: 'OCP.AIS.AIS_MultipleConnectedInteractive' object has no attribute 'Shape'
```
---

Fragile point also here. The export context menu actions probably don't work for non-shapes?

https://github.com/CadQuery/CQ-editor/blob/0d091ba3d20114b4815f796ec48ab33f4ebbeb41/cq_editor/widgets/object_tree.py#L353

---

MWE

```
from cadquery import cq
from OCP.AIS import AIS_Line
from OCP.Geom import Geom_CartesianPoint

box = cq.Workplane().box(10, 10, 10)

line = AIS_Line(
Geom_CartesianPoint(0, 0, 0),
Geom_CartesianPoint(10, 10, 10)
)

show_object(box, 'box')
show_object(line, 'line')
```

---

Other direction seems to work fine (= if I select an AIS_MultipleConnectedInteractive from the tree, it gets highlighted in the viewer correctly)