Refactor model and root handling

[philippjfr]

This PR tries to minimize the amount of nesting widgets and panes use, i.e. objects that can be updated inplace do not need to be nested inside a container. Only objects where the bokeh model is not well defined and therefore may need to be replaced entirely need to be nested.

This makes testing simpler, makes the bokeh model structure match the panel structure more closely and makes things easier for bokeh's layouts.

[codecov-io]

Codecov Report

Merging #259 into master will decrease coverage by 0.31%.
The diff coverage is 97.18%.

@@            Coverage Diff             @@
##           master     #259      +/-   ##
==========================================
- Coverage   89.12%   88.81%   -0.32%     
==========================================
  Files          34       34              
  Lines        5436     5336     -100     
==========================================
- Hits         4845     4739     -106     
- Misses        591      597       +6

Impacted Files	Coverage Δ
panel/interact.py	73.38% <0%> (+0.27%)	⬆️
panel/tests/test_vega.py	96.61% <100%> (-0.36%)	⬇️
panel/tests/test_panes.py	94.36% <100%> (-1.12%)	⬇️
panel/vega.py	83.33% <100%> (+0.47%)	⬆️
panel/tests/test_interact.py	100% <100%> (ø)	⬆️
panel/tests/test_layout.py	99.16% <100%> (-0.84%)	⬇️
panel/widgets.py	94.02% <100%> (-0.1%)	⬇️
panel/plotly.py	100% <100%> (ø)	⬆️
panel/tests/test_param.py	99.61% <100%> (ø)	⬆️
panel/tests/test_plotly.py	97.59% <100%> (-0.24%)	⬇️
... and 6 more

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