Inlined subobject expand toggle button

[philippjfr]

Fixes #308

[codecov-io]

Codecov Report

Merging #329 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #329      +/-   ##
==========================================
+ Coverage   89.53%   89.54%   +0.01%     
==========================================
  Files          84       84              
  Lines        6784     6811      +27     
==========================================
+ Hits         6074     6099      +25     
- Misses        710      712       +2

Impacted Files	Coverage Δ
panel/param.py	92.18% <100%> (+0.06%)	⬆️
panel/tests/test_param.py	100% <100%> (ø)	⬆️
panel/pane/markup.py	92.7% <0%> (-0.35%)	⬇️

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