Allow setting horizontal and vertical alignment separately

[philippjfr]

Somehow missed this entirely, the align property in bokeh has always supported providing a (horizontal, vertical) alignment tuple. This merely exposes this ability in Panel.

[codecov]

Codecov Report

Merging #2072 (3219a03) into master (01869de) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2072      +/-   ##
==========================================
- Coverage   84.43%   84.39%   -0.04%     
==========================================
  Files         178      178              
  Lines       20636    20636              
==========================================
- Hits        17423    17415       -8     
- Misses       3213     3221       +8     

Impacted Files	Coverage Δ
panel/viewable.py	72.38% <100.00%> (ø)
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%)	⬇️

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