Add GUI elements to fig.Figure

[adam-urbanczyk]

This PR adds a toolbar and minimal objects list to fig.Figure:

[adam-urbanczyk]

@lorenzncode this is by far not finished, but I remember you were interested in this.

[codecov]

Codecov Report

❌ Patch coverage is 70.64220% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.39%. Comparing base (fea85a5) to head (fca2061).

Files with missing lines	Patch %	Lines
cadquery/fig.py	70.64%	30 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1943      +/-   ##
==========================================
- Coverage   95.77%   95.39%   -0.39%     
==========================================
  Files          29       29              
  Lines        7856     7919      +63     
  Branches     1183     1188       +5     
==========================================
+ Hits         7524     7554      +30     
- Misses        192      223      +31     
- Partials      140      142       +2     

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