Improved behavior for fixed widths and heights

[philippjfr]

This PR forces a fixed height to be respected if specified either by setting sizing_mode='fixed' or by setting the corresponding min_width or min_height property at the same time.

Fixes #23

[jsignell]

Fixes the issue I was seeing locally thanks!

[codecov-io]

Codecov Report

Merging #258 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #258      +/-   ##
==========================================
+ Coverage   89.31%   89.32%   +0.01%     
==========================================
  Files          44       44              
  Lines        5437     5445       +8     
==========================================
+ Hits         4856     4864       +8     
  Misses        581      581

Impacted Files	Coverage Δ
panel/layout.py	95.63% <100%> (-0.13%)	⬇️
panel/tests/test_reactive.py	100% <100%> (ø)	⬆️
panel/tests/test_widgets.py	100% <100%> (ø)	⬆️
panel/viewable.py	72.52% <100%> (+1.59%)	⬆️
panel/widgets.py	94.42% <100%> (-0.03%)	⬇️

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