Refactor the logic inside keys_display_bindings_bar() and remove the
need to place the "show next page" key binding at the right positions.
This used to be a pain to maintain, since we always had to move key
bindings around when introducing a new key. Fix this by passing the
actual key bindings in an array and using a separate parameter for the
"show next page" key binding (which is automatically inserted at the
right places from now on).
Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>