feat: ViewListener to get dimensions of DOM elements in all views

[maartenbreddels]

@astrofrog I did not test this a lot, but you can try out if this works as you expect:

from bqplot_image_gl.viewlistener import ViewListener
import ipywidgets as widgets
slider = widgets.FloatSlider()
slider
vl = ViewListener(widget=slider, css_selector=".ui-slider-handle")

[codecov]

Codecov Report

Merging #57 (249fdf5) into master (9d0a827) will decrease coverage by 8.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
- Coverage   58.00%   50.00%   -8.00%     
==========================================
  Files           5        6       +1     
  Lines         100      116      +16     
==========================================
  Hits           58       58              
- Misses         42       58      +16     

Impacted Files	Coverage Δ
bqplot_image_gl/viewlistener.py	0.00% <0.00%> (ø)

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[astrofrog]

Nice! I'll play with this to see if it works nicely for determining aspect ratios.

[maartenbreddels]

For bqplot, this seems to work well:

vl = ViewListener(widget=figure, css_selector=".svg-figure > g")

As you can see in the screenshot:

the second viewlistener has the correct dimensions.

Not sure why the JS exception occurs.

[maartenbreddels]

After some bug hunting and polishing, I think this is ready to go.

[astrofrog]

This seems to work nicely! I think we should just merge, and I'll open an issue if I run into problems when implementing the aspect ratio stuff in glupyter.