Refactor callbacks to be initialized on instantiation

[philippjfr]

Instead of creating callbacks per bokeh model this PR creates single callbacks on instantiation which update all existing models.

Fixes #292

• Ensure all Select widgets allow both list and dict
• Update ParamMethod pane
• Update CrossSelector
• Simplify _callbacks
• Remove final flag on cleanup
• Ensure correct parameters are watched
• Write tests for HoloViews pane, updating of backend parameter, more tests
• Add dedicated cleanup tests

[codecov-io]

Codecov Report

Merging #293 into master will increase coverage by 0.18%.
The diff coverage is 90.17%.

@@            Coverage Diff             @@
##           master     #293      +/-   ##
==========================================
+ Coverage   88.76%   88.94%   +0.18%     
==========================================
  Files          60       61       +1     
  Lines        5998     6117     +119     
==========================================
+ Hits         5324     5441     +117     
- Misses        674      676       +2

Impacted Files	Coverage Δ
panel/io.py	38% <0%> (ø)	⬆️
panel/tests/test_vega.py	96.61% <100%> (ø)	⬆️
panel/tests/test_plotly.py	97.46% <100%> (-0.13%)	⬇️
panel/tests/test_interact.py	100% <100%> (ø)	⬆️
panel/tests/test_layout.py	100% <100%> (+0.34%)	⬆️
panel/tests/test_param.py	99.65% <100%> (+0.03%)	⬆️
panel/widgets/base.py	100% <100%> (ø)	⬆️
panel/pane/equation.py	83.58% <100%> (+0.24%)	⬆️
panel/tests/test_widgets.py	100% <100%> (ø)	⬆️
panel/tests/test_reactive.py	100% <100%> (ø)	⬆️
... and 28 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8e0ef61...3792684. Read the comment docs.