Added implementation of lines in scatter viewer

[astrofrog]

This adds support for showing lines in the scatter viewer.

This currently doesn't support showing variable colors along the line but I'm not sure if this is possible yet (bqplot/bqplot-gl#18)

cc @kecnry

[codecov]

Codecov Report

Patch coverage: 74.35% and project coverage change: -0.11% ⚠️

Comparison is base (60f58c3) 86.84% compared to head (177a77b) 86.73%.
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #398      +/-   ##
==========================================
- Coverage   86.84%   86.73%   -0.11%     
==========================================
  Files          89       89              
  Lines        5077     5111      +34     
==========================================
+ Hits         4409     4433      +24     
- Misses        668      678      +10     

Files Changed	Coverage Δ
glue_jupyter/bqplot/scatter/layer_artist.py	87.50% <65.51%> (-3.51%)	⬇️
glue_jupyter/bqplot/compatibility.py	83.33% <100.00%> (+8.33%)	⬆️
glue_jupyter/bqplot/scatter/tests/test_viewer.py	100.00% <100.00%> (ø)
glue_jupyter/common/state_widgets/layer_scatter.py	87.50% <100.00%> (+0.83%)	⬆️

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[kecnry]

Seems to work great, thanks!