Improve navigation

Until now one could navigate only with Up/Down (and aliases).
It'd be nice to be able to use Page-up/down, number, search?

Proposal:
- P - N to navigate by page,
- p - n to navigate by track (as is now)
- /text - highlight all tracks containing text and make p/n traverse through them.
  ESC to cancel
- :n go to _n_th track
