Compatibility for new bokeh layout system

[philippjfr]

Currently bokeh layouts are going through a major refactor. This PR will try to maintain compatibility with that bokeh branch (bokeh/bokeh#8085) while staying up to date with panel master.

Things that need fixing:

• Custom (Vega/Plotly) models broken
• Widget sizing completely broken
• Widgets (specifically sliders and buttons) often do not allocate enough vertical space especially when displayed on their own (need at least some padding)
• Tabs are completely broken
• Column/Rows do not update when children are modified
• Widgets don't line up even if given same width/height
• Button.clicks attribute was removed
• "bk-widgetbox" css class is very ugly and does not improve spacing issues mentioned above
• Widgets do not update in response to changes in their height/width

I'll be adding new items as I discover them and ticking off issues when they're fixed

[DancingQuanta]

Trying out Bokeh version 1.1.0dev2 with panel.Pipeline and changing the transition buttons' widths

pipeline = pn.pipeline.Pipeline(stages, debug=True)

# Change width of buttons
pipeline.layout[0][1]._widget_box.width = 100
pipeline.layout[0][2]._widget_box.width = 100

pipeline.layout.servable()

Modifying the widths have no effect with this version of bokeh.

[philippjfr]

This PR is pretty outdated, I'll probably push an entirely new PR for compatibility fixes and adding new features.

[philippjfr]

Modifying the widths have no effect with this version of bokeh.

I've already raised this issue upstream with bokeh.

[DancingQuanta]

Could you give the reference to bokeh issue please?

[philippjfr]

I'm going to merge this and further development can happen on new PRs.

[codecov-io]

Codecov Report

Merging #32 into master will decrease coverage by 0.07%.
The diff coverage is 98.78%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
- Coverage   89.38%   89.31%   -0.08%     
==========================================
  Files          44       44              
  Lines        5474     5437      -37     
==========================================
- Hits         4893     4856      -37     
  Misses        581      581

Impacted Files	Coverage Δ
setup.py	0% <ø> (ø)	⬆️
panel/tests/test_interact.py	100% <100%> (ø)	⬆️
panel/tests/test_layout.py	100% <100%> (ø)	⬆️
panel/tests/test_param.py	99.61% <100%> (-0.02%)	⬇️
panel/holoviews.py	83.41% <100%> (ø)	⬆️
panel/pipeline.py	80.82% <100%> (ø)	⬆️
panel/param.py	90.74% <100%> (+0.19%)	⬆️
panel/tests/test_widgets.py	100% <100%> (ø)	⬆️
panel/widgets.py	94.45% <100%> (+0.04%)	⬆️
panel/viewable.py	70.93% <100%> (+1.25%)	⬆️
... and 6 more

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