Add FloatPanel layout

[philippjfr]

Implements #2745

[codecov]

Codecov Report

Merging #4707 (931ced5) into main (d4435b9) will decrease coverage by 0.29%.
The diff coverage is 91.42%.

@@            Coverage Diff             @@
##             main    #4707      +/-   ##
==========================================
- Coverage   83.25%   82.96%   -0.29%     
==========================================
  Files         267      268       +1     
  Lines       37624    37658      +34     
==========================================
- Hits        31323    31243      -80     
- Misses       6301     6415     +114     

Flag	Coverage Δ
ui-tests	39.36% <91.17%> (-1.23%)	⬇️
unitexamples-tests	73.22% <91.42%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
panel/__init__.py	100.00% <ø> (ø)
panel/config.py	68.69% <ø> (-0.46%)	⬇️
panel/layout/gridstack.py	85.07% <ø> (ø)
panel/reactive.py	80.75% <0.00%> (-0.28%)	⬇️
panel/layout/float.py	93.93% <93.93%> (ø)
panel/layout/__init__.py	100.00% <100.00%> (ø)

... and 13 files with indirect coverage changes

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