Fix updating of selection property

[philippjfr]

Could not reset the selection in a callback since the _updating check prevented changes.

[codecov]

Codecov Report

Merging #2128 (51a6d81) into master (43de0fa) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2128      +/-   ##
==========================================
- Coverage   84.27%   84.24%   -0.03%     
==========================================
  Files         182      182              
  Lines       21415    21413       -2     
==========================================
- Hits        18047    18040       -7     
- Misses       3368     3373       +5     

Impacted Files	Coverage Δ
panel/reactive.py	77.30% <ø> (+0.08%)	⬆️
panel/tests/pane/test_base.py	88.23% <0.00%> (-11.77%)	⬇️
panel/pane/base.py	87.89% <0.00%> (-0.90%)	⬇️

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