Displaying wires properly when zoomed out

[Ben1152000]

When the time window is too large (see sootty example/example3.vcd -w "pc"), the data can become cluttered and in some cases, unreadable. If there is too much data to show, it should still be possible to visualize it in a way that presents what's going on. This might include selecting "key frames" that are representative of the general behavior, or adding a marker that informs the user that they need to zoom in closer.