If a timer expires while stopped, it should trigger when TIMER ON is
run. Instead, on QB64 it triggers randomly after the TIMER ON happens.
The basic issue is that `qbevent` needs to be set to trigger the timer,
but TIMER ON doesn't do that. The regular timer logic that does that
already set it when the timer expired while sleeping, so it won't set it
again. The simplest solution is to just alway set qbevent = 1 when TIMER
ON is done. It's slightly less efficent but doesn't hurt to set it even
when there are no timers that expired.
Fixes: #293
The command Sleep is supposed to allow timers to trigger while the
program is sleeping on the delay. This is achieved in QB64 by having
commands that do delays manually call evnt() to trigger events if they
come up (of which timers are one).
Sleep has a custom implementation for console programs on Windows which
doesn't do this, so I redid the logic so that it calls evnt() at regular
intervals while waiting for input. Additionally, due to now calling
evnt() we also need to check if we should exit sleep early due to an
evnt() firing.
Fixes: #294
