Improved logic to avoid events boomeranging

[philippjfr]

The _expecting flag which was an attempt to ensure that events don't bounce around was not actually doing anything. Additionally the bi-directional link between a Parameterized and a widget was also not handled correctly and is now replaced by a _updating flag on the Param pane.

[codecov-io]

Codecov Report

Merging #288 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #288      +/-   ##
==========================================
- Coverage   88.67%   88.63%   -0.04%     
==========================================
  Files          60       60              
  Lines        6002     6000       -2     
==========================================
- Hits         5322     5318       -4     
- Misses        680      682       +2

Impacted Files	Coverage Δ
panel/param.py	90.39% <100%> (+0.74%)	⬆️
panel/viewable.py	72.75% <100%> (-0.06%)	⬇️
panel/util.py	66.12% <0%> (-1.21%)	⬇️
panel/pane/image.py	81.11% <0%> (-1.12%)	⬇️
panel/interact.py	73.03% <0%> (-0.75%)	⬇️

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