Feature: Provide keyboard help while playing

[forthrin]

https://github.com/libsidplayfp/sidplayfp/blob/master/src/menu.cpp#L55

←/→  Previous/Next  1-9  Toggle voices  h    Help
↓/↑  Standard/Fast  a-d  Toggle sample  q    Quit
p    Pause          f    Toggle filter

Notices and accompanied suggestions:

1. Move toggle sample to a-d to free up q for quit. Fits snugly with related f for toggle filter.
2. Keep esc as an implicit and expected way to quit.