input.conf: remap pgup/dwn to chapter seeks

As discussed in #973.

Keep the old bindings for now; there's no reason to unmap them yet.
player: indicate on the status line whether a seek is active

This is delayed by 300ms - before that, the status doesn't change. I
feel like it would too annoying if the status line would "flicker" on
normal seek by quickly showing and hiding the indicator.
command: add a "seeking" property

The client API exports this state via events already, but maybe it's
better to explicitly provide this property in order to facilitate use on
OSD and similar cases.