Add more visuals and cell execution

[adam-urbanczyk]

• Location, plane --> trihedron
• Initial support for cell execution
• overwrite cq.vis.show when executing
• format
• fix selection
• add some type hints
• fix core dump on appveyor/linux
• tests

[codecov]

Codecov Report

Attention: Patch coverage is 85.15152% with 98 lines in your changes missing coverage. Please review.

Project coverage is 87.16%. Comparing base (089bb86) to head (cc801ce).

Files with missing lines	Patch %	Lines
cq_editor/cq_utils.py	68.60%	19 Missing and 8 partials ⚠️
cq_editor/utils.py	72.97%	6 Missing and 4 partials ⚠️
cq_editor/preferences.py	40.00%	9 Missing ⚠️
cq_editor/widgets/occt_widget.py	76.31%	8 Missing and 1 partial ⚠️
cq_editor/widgets/viewer.py	86.95%	7 Missing and 2 partials ⚠️
cq_editor/main_window.py	90.69%	8 Missing ⚠️
cq_editor/widgets/debugger.py	91.57%	7 Missing and 1 partial ⚠️
cq_editor/widgets/editor.py	89.58%	1 Missing and 4 partials ⚠️
cq_editor/widgets/object_tree.py	95.65%	2 Missing and 2 partials ⚠️
cq_editor/mixins.py	86.95%	2 Missing and 1 partial ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #452      +/-   ##
==========================================
- Coverage   89.12%   87.16%   -1.97%     
==========================================
  Files          19       19              
  Lines        1582     1659      +77     
  Branches      190      186       -4     
==========================================
+ Hits         1410     1446      +36     
- Misses        139      168      +29     
- Partials       33       45      +12     

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