RadioList: pagedown/pageup support

[gpotter2]

This PR:

• was part of RadioList: search #758
• contains support for the PageUp / PageDown keys on RadioList

[codecov-io]

Codecov Report

Merging #761 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #761      +/-   ##
==========================================
- Coverage      71%   70.97%   -0.04%     
==========================================
  Files         144      144              
  Lines       13618    13624       +6     
==========================================
  Hits         9669     9669              
- Misses       3949     3955       +6

Impacted Files	Coverage Δ
prompt_toolkit/widgets/base.py	29.11% <0%> (-0.69%)	⬇️

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[jonathanslenders]

This has been merged here: 6f11f1a
Thanks a lot!