Network File and Explorer: Load data for edges; show colored and labelled edges

[janezd]

Issue

Resolves #247.

Description of changes

TO DO

• Documentation, especially for Network File

Includes

• Code changes
• Tests
• Documentation

[codecov-commenter]

Codecov Report

Patch coverage: 92.70% and project coverage change: +4.06 🎉

Comparison is base (3496707) 73.34% compared to head (eb3ad0e) 77.40%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #249      +/-   ##
==========================================
+ Coverage   73.34%   77.40%   +4.06%     
==========================================
  Files          21       26       +5     
  Lines        2975     3581     +606     
==========================================
+ Hits         2182     2772     +590     
- Misses        793      809      +16     

Impacted Files	Coverage Δ
...gecontrib/network/network/tests/test_embeddings.py	100.00% <ø> (ø)
...rangecontrib/network/network/tests/test_twomode.py	100.00% <ø> (ø)
orangecontrib/network/widgets/graphview.py	66.43% <60.00%> (-0.60%)	⬇️
orangecontrib/network/widgets/OWNxExplorer.py	67.60% <73.33%> (-0.07%)	⬇️
orangecontrib/network/widgets/OWNxFile.py	95.62% <97.57%> (+9.36%)	⬆️
orangecontrib/network/network/base.py	81.08% <100.00%> (ø)
orangecontrib/network/network/compose.py	100.00% <100.00%> (ø)
orangecontrib/network/network/readwrite.py	82.43% <100.00%> (+1.00%)	⬆️
...rangecontrib/network/network/tests/test_compose.py	100.00% <100.00%> (ø)
...ngecontrib/network/network/tests/test_readwrite.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[janezd]

TODO: Remove. :)