Start implementing 'always-on' MouseInteraction

[astrofrog]

@maartenbreddels - this is what I have so far but I have run into an issue which is that the print statement to show when mouse interactions are received only works if you first select a tool, e.g. rectangular selection. For some reason it doesn't work before this, when next is set to None.

Note that there is also a default 'click and drag ROI' mouse mode that gets selected when other tools aren't selected - this doesn't seem to play nicely with having a MouseInteration that has a next which is the ROI MouseInteraction, so this will also require some investigation.

Note that this requires glue-viz/bqplot-image-gl#46

[codecov]

Codecov Report

Merging #213 (bee7c4d) into master (ce94a93) will decrease coverage by 0.09%.
The diff coverage is 86.20%.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
- Coverage   90.57%   90.47%   -0.10%     
==========================================
  Files          83       82       -1     
  Lines        3988     3959      -29     
==========================================
- Hits         3612     3582      -30     
- Misses        376      377       +1     

Impacted Files	Coverage Δ
glue_jupyter/bqplot/common/viewer.py	89.58% <69.23%> (-3.19%)	⬇️
glue_jupyter/bqplot/common/tools.py	75.22% <100.00%> (ø)
glue_jupyter/bqplot/histogram/layer_artist.py	91.66% <100.00%> (ø)
glue_jupyter/bqplot/profile/layer_artist.py	93.20% <100.00%> (ø)
glue_jupyter/bqplot/scatter/layer_artist.py	99.42% <100.00%> (ø)
glue_jupyter/bqplot/tests/test_bqplot.py	100.00% <100.00%> (ø)
glue_jupyter/common/state3d.py	81.69% <100.00%> (ø)
glue_jupyter/ipyvolume/scatter/layer_artist.py	98.00% <100.00%> (ø)
glue_jupyter/ipyvolume/volume/layer_artist.py	95.91% <100.00%> (ø)
glue_jupyter/link.py	56.25% <100.00%> (ø)
... and 4 more

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[maartenbreddels]

@astrofrog would be great if you can try again with glue-viz/bqplot-image-gl#47

[astrofrog]

@maartenbreddels - the CI is being a bit slow to start but this is ready for review.

[astrofrog]

(I just rebased this)

[astrofrog]

This seems to work fine for spacetelescope/jdaviz#429

[maartenbreddels]

Looks good, the only 'downside' i see is that we'll always get a lot of events. We could possibly add an extra argument to the add_event_callback which events we want to listen to, and set _mouse_interact.events based on that. But, we can postpone that for now.

[astrofrog]

@maartenbreddels - yes good idea. I'll merge this for now and will add something like this in a follow-up PR.

[pllim]

Is this released? If not what is the timeline? This is blocking spacetelescope/jdaviz#429