Add more Qt mouse button names

[jwiggins]

Fixes #334

[codecov-io]

Codecov Report

Merging #345 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #345   +/-   ##
=======================================
  Coverage   33.94%   33.94%           
=======================================
  Files         206      206           
  Lines       18290    18290           
  Branches     2414     2414           
=======================================
  Hits         6209     6209           
  Misses      11688    11688           
  Partials      393      393

Impacted Files	Coverage Δ
enable/qt4/constants.py	90.47% <ø> (ø)	⬆️

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[jwiggins]

@corranwebster Care to comment on the names I picked for the buttons?

[corranwebster]

Names look fine to me - I've never had a 5 button mouse, but that seems consistent usage with searches.

[jwiggins]

Thanks for the review!