Add refresh functionality to FileSelector

[philippjfr]

Implements #1961

[codecov]

Codecov Report

Merging #1962 (5456570) into master (2d6d043) will increase coverage by 0.00%.
The diff coverage is 64.00%.

@@           Coverage Diff           @@
##           master    #1962   +/-   ##
=======================================
  Coverage   84.87%   84.87%           
=======================================
  Files         154      154           
  Lines       18993    19014   +21     
=======================================
+ Hits        16121    16139   +18     
- Misses       2872     2875    +3     

Impacted Files	Coverage Δ
panel/widgets/file_selector.py	85.09% <64.00%> (-4.20%)	⬇️
panel/io/callbacks.py	36.84% <0.00%> (+7.89%)	⬆️

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