Feat: add native non-blocking vtk viewer to vis.py (#1902)

[praxcom]

• Uses native vtk viewer in spwned thread

• No use of browser or server

• Message passing via queue standard library

• Interacts very similarly to existing vis.show func

• Added example script EX102_Live_Vis.py, which works perfectly but no pure tests

[kurtsansom]

duplicate line here.

[adam-urbanczyk]

Probably better to make this into an extension outside of the CQ codebase. At least for the time being.

[praxcom]

Leaving as extension is fine if that's what you think is best, but can I ask why?
I needed this feature for embedded cadquery in a in-house gui program so didnt want to use cq-edit.
I can imagine the use case being applicable to others too, hence creating the pull request.

The proposed use case as per Issue #1902:

import cadquery as cq
from cadquery.vis import show_live

box = cq.Workplane().box(5, 2, 1).val() #initial geometry

viewer = show_live(box) #initial view

def gui_callback(event):
    # Do stuff...
    box = #...updated geometry 
    viewer.update(box) #update the view, no blocking on either side

### GUI CODE + LOOP

[adam-urbanczyk]

Mostly to limit the maintenance burden. If this method turns out to be reliable, we could always replace the current show later.