Clean up input events.

Clean up input events.

A new variable intf-event has been added.
It supercedes the following variables (they are still present for the
time being):
 - intf-change
 - intf-change-vout
 - rate-change
 - stats-change
and allows to have a better knowledge of the change done.

It is documented in vlc_input.h (in progress).