Fixed issue setting (Multi)Select value when options changed

[philippjfr]

This fixes callbacks set on a MultiSelect widget which change the values. Due to the ordering of the watchers it can happen that the options change before the value change event has been propagated to the underlying bokeh model. Ideally we would have a way to elevate the precedence of watch calls which would allow us to order the watchers appropriately.

This PR simply addresses the symptom by skipping values that aren't found.

[codecov-io]

Codecov Report

Merging #291 into master will increase coverage by 0.03%.
The diff coverage is 96.42%.

@@            Coverage Diff             @@
##           master     #291      +/-   ##
==========================================
+ Coverage   88.73%   88.76%   +0.03%     
==========================================
  Files          60       60              
  Lines        5973     5998      +25     
==========================================
+ Hits         5300     5324      +24     
- Misses        673      674       +1

Impacted Files	Coverage Δ
panel/tests/test_widgets.py	100% <100%> (ø)	⬆️
panel/widgets/select.py	97.12% <87.5%> (-0.42%)	⬇️

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