Always wrap Div and widgets in WidgetBox

[philippjfr]

Makes the handling of Div and widget bokeh models more consistent by always wrapping them in a widget box, which inherits width/height/sizing_mode from the model.

[codecov-io]

Codecov Report

Merging #25 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   92.68%   92.71%   +0.02%     
==========================================
  Files          19       19              
  Lines        2242     2236       -6     
==========================================
- Hits         2078     2073       -5     
+ Misses        164      163       -1

Impacted Files	Coverage Δ
panel/tests/test_panes.py	97.9% <100%> (-0.07%)	⬇️
panel/util.py	77.88% <100%> (+0.74%)	⬆️
panel/pane.py	82.52% <100%> (+0.07%)	⬆️

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