Fix #345 - add scroll wheel support

[peterbrittain]

Merge!

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 97.53%. Comparing base (ce7523b) to head (ec15f88).

Files with missing lines	Patch %	Lines
asciimatics/widgets/frame.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #394   +/-   ##
=======================================
  Coverage   97.53%   97.53%           
=======================================
  Files          53       53           
  Lines        5881     5893   +12     
=======================================
+ Hits         5736     5748   +12     
  Misses        145      145           

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