DecodeTrace/DecodeSignal: Rework trace drawing interval

DecodeTrace/DecodeSignal: Rework trace drawing interval

Before, we'd redraw the decode trace every n annotations that
came in. However, that causes a lot of unnecessary redraws
when we're zoomed out relatively far and makes the UI less
responsive in return.

Instead, use a one-shot timer with a fixed delay to redraw
the trace after data has been processed. This way, there is
an upper bound to how often we redraw the trace while at the
same time providing a fixed interval at which we update,
regardless of how many annotations came in, making the
trace fill up continuously.

With that in place, we can also remove annotation_count_
and inc_annotation_count().