Ensure ListPanel objects can be passed as keyword

[philippjfr]

Passing objects as a keyword was not previously supported.

[codecov]

Codecov Report

Merging #1502 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1502      +/-   ##
==========================================
- Coverage   86.32%   86.30%   -0.02%     
==========================================
  Files         142      142              
  Lines       15769    15970     +201     
==========================================
+ Hits        13612    13783     +171     
- Misses       2157     2187      +30     

Impacted Files	Coverage Δ
panel/layout/base.py	91.23% <100.00%> (+0.04%)	⬆️
panel/tests/layout/test_base.py	100.00% <100.00%> (ø)
panel/io/location.py	93.87% <0.00%> (+1.77%)	⬆️
panel/template/base.py	71.42% <0.00%> (+2.03%)	⬆️
panel/viewable.py	74.63% <0.00%> (+4.32%)	⬆️

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