Tidy up how layout is defined for viewers and provide a way to customize

[astrofrog]

This tidies up a lot of the code that does the layout for the viewers to be defined in the base class for all Jupyter-based viewers. In addition, this defines a public API for accessing the components for any given viewer, and provides a way to register a custom 'layout factory' that defines a new default layout for viewers, for example:

from ipywidgets import VBox
from glue_jupyter import set_layout_factory

def simple_viewer_layout(viewer):
    return VBox([viewer.toolbar_selection_tools, viewer.figure_widget])

set_layout_factory(simple_viewer_layout)

This will make it easier to experiment with ipyvuetify layouts.

Finally, this creates a new widget for editing layer options which consists of a way to select the current layer (for now a dropdown) and shows the options for that specific layer. In future we can improve this, but at least this provides a short-term fix for #84 by avoiding having many tabs when there are a lot of layers present.

[codecov]

Codecov Report

Merging #108 into master will decrease coverage by 0.21%.
The diff coverage is 89.62%.

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
- Coverage   92.15%   91.94%   -0.22%     
==========================================
  Files          68       69       +1     
  Lines        2946     2980      +34     
==========================================
+ Hits         2715     2740      +25     
- Misses        231      240       +9

Impacted Files	Coverage Δ
glue_jupyter/matplotlib/base.py	100% <100%> (ø)	⬆️
glue_jupyter/bqplot/tests/test_bqplot.py	100% <100%> (ø)	⬆️
..._jupyter/ipyvolume/common/viewer_options_widget.py	100% <100%> (ø)	⬆️
glue_jupyter/ipyvolume/tests/test_ipyvolume.py	100% <100%> (ø)	⬆️
glue_jupyter/ipyvolume/volume/viewer.py	100% <100%> (ø)	⬆️
...lue_jupyter/common/state_widgets/viewer_profile.py	100% <100%> (ø)	⬆️
...e_jupyter/common/state_widgets/viewer_histogram.py	100% <100%> (ø)	⬆️
glue_jupyter/ipyvolume/scatter/viewer.py	100% <100%> (ø)	⬆️
glue_jupyter/bqplot/common/viewer.py	85.5% <100%> (-2.59%)	⬇️
glue_jupyter/ipyvolume/common/tools.py	97.29% <100%> (ø)	⬆️
... and 9 more

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