Add PDF pane

[philippjfr]

Implements #1117

[codecov]

Codecov Report

Merging #2444 (45294b6) into master (abc56e7) will decrease coverage by 0.00%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #2444      +/-   ##
==========================================
- Coverage   83.47%   83.47%   -0.01%     
==========================================
  Files         182      182              
  Lines       22668    22702      +34     
==========================================
+ Hits        18923    18951      +28     
- Misses       3745     3751       +6     

Impacted Files	Coverage Δ
panel/pane/image.py	78.33% <81.25%> (+1.93%)	⬆️
panel/pane/__init__.py	100.00% <100.00%> (ø)
panel/pane/plot.py	57.75% <100.00%> (ø)
panel/tests/pane/test_image.py	100.00% <100.00%> (ø)
panel/viewable.py	68.29% <0.00%> (-1.11%)	⬇️

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