New timing infrastructure. There's a new function schedule_timer()

New timing infrastructure. There's a new function schedule_timer()
which pretty much any module can call to request a call-back in the
future. So terminal.c can do its own handling of blinking, visual
bells and deferred screen updates, without having to rely on
term_update() being called 50 times a second (fixes: pterm-timer);
and ssh.c and telnet.c both invoke a new module pinger.c which takes
care of sending keepalives, so they get sent uniformly in all front
ends (fixes: plink-keepalives, unix-keepalives).

git-svn-id: svn://svn.tartarus.org/sgt/putty@4906 cda61777-01e9-0310-a592-d414129be87e