Cleaned up Panes and added tests

[philippjfr]

• Cleaned up Pane.applies implementations
• Add Pane tests

[codecov-io]

Codecov Report

Merging #18 into master will increase coverage by 7.15%.
The diff coverage is 94.2%.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
+ Coverage   85.06%   92.21%   +7.15%     
==========================================
  Files          15       17       +2     
  Lines        1808     2069     +261     
==========================================
+ Hits         1538     1908     +370     
+ Misses        270      161     -109

Impacted Files	Coverage Δ
setup.py	0% <ø> (ø)	⬆️
panel/tests/test_util.py	100% <100%> (ø)
panel/tests/fixtures.py	100% <100%> (ø)	⬆️
panel/util.py	77.14% <66.66%> (+26.18%)	⬆️
panel/panes.py	82.57% <66.66%> (+40.49%)	⬆️
panel/tests/test_panes.py	97.93% <97.93%> (ø)

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