Renamed precedence to priority and deleted trailing whitespace

[jlstevens]

PR title says it all. I find using precedence for panes horribly confusing:

1. Panel uses parameters with defined precedence internally.
2. Panel sorts on parameter precedence when building widgets for parameterized objects.
3. Panel applies panes according to its own notion of precedence.

Now if you grep for precedence, you know that the only concept there relates to parameters.

[codecov-io]

Codecov Report

Merging #235 into master will not change coverage.
The diff coverage is 93.75%.

@@           Coverage Diff           @@
##           master     #235   +/-   ##
=======================================
  Coverage   90.02%   90.02%           
=======================================
  Files          34       34           
  Lines        5351     5351           
=======================================
  Hits         4817     4817           
  Misses        534      534

Impacted Files	Coverage Δ
panel/tests/test_pipeline.py	99.27% <ø> (ø)	⬆️
panel/vega.py	82.85% <100%> (ø)	⬆️
panel/param.py	90.46% <100%> (ø)	⬆️
panel/plotly.py	100% <100%> (ø)	⬆️
panel/holoviews.py	84.1% <100%> (ø)	⬆️
panel/pane.py	82.5% <91.66%> (ø)	⬆️

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