Dequeued stimuli are never removed from display stack

The "dequeue_stimulus" action makes the target stimulus invisible, but it doesn't remove it from the StimulusDisplay's display stack. Instead, the stimulus stays in the stack indefinitely, even after the experiment is stopped and restarted.

This isn't necessarily a huge deal, but it definitely wastes CPU cycles. Specifically, StimulusDisplay::drawDisplayStack will test every stimulus that has ever been displayed for visibility, even if the user only presents one stimulus at a time and dutifully dequeues each one after display.

Probably the right thing is for StimulusDisplay::updateDisplay to remove any stimuli that become invisible.