Allow setting root_directory on FileSelector widget

[philippjfr]

Closes #2051

[codecov]

Codecov Report

Merging #2086 (58a5a4c) into master (650c9e7) will decrease coverage by 0.01%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##           master    #2086      +/-   ##
==========================================
- Coverage   84.40%   84.39%   -0.02%     
==========================================
  Files         178      179       +1     
  Lines       20646    20764     +118     
==========================================
+ Hits        17427    17523      +96     
- Misses       3219     3241      +22     

Impacted Files	Coverage Δ
panel/widgets/file_selector.py	84.52% <72.72%> (-0.57%)	⬇️
panel/tests/pane/test_base.py	88.23% <0.00%> (-11.77%)	⬇️
panel/io/reload.py	64.36% <0.00%> (-2.30%)	⬇️
panel/pane/base.py	90.61% <0.00%> (-0.94%)	⬇️
panel/compiler.py	7.17% <0.00%> (-0.35%)	⬇️
panel/tests/test_depends.py	100.00% <0.00%> (ø)
panel/tests/pane/test_perspective.py	100.00% <0.00%> (ø)
panel/template/base.py	80.33% <0.00%> (+0.47%)	⬆️
panel/depends.py	83.82% <0.00%> (+0.49%)	⬆️
panel/pane/perspective.py	91.80% <0.00%> (+2.51%)	⬆️

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