companion: Add support for several buttons

[postlund]

WIP

Adds support for a few buttons. Still haven't found any equivalent to the remaining buttons.

Still missing tests and documentation.

Relates to #1190

[codecov]

Codecov Report

Merging #1218 (c5a1155) into master (e1dab57) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1218      +/-   ##
==========================================
- Coverage   95.25%   95.22%   -0.04%     
==========================================
  Files          58       58              
  Lines        6148     6172      +24     
==========================================
+ Hits         5856     5877      +21     
- Misses        292      295       +3     

Impacted Files	Coverage Δ
pyatv/companion/__init__.py	91.07% <100.00%> (+0.79%)	⬆️
pyatv/raop/raop.py	89.07% <0.00%> (-0.57%)	⬇️

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