Added filterPoints option to WaveformView

[tnibler]

This allows the user to provide a function that controls whether a particular point is displayed on the Waveform. Main use case: there are a lot of points on the overview, and only a small part of them are actually important to see there.

What might be missing here is something to emit points.update in case the user changes something in their filter function and visible points need to be recomputed.

You don't need to merge this btw, I'm just adding stuff I need here and there, so if you don't want things upstream no problem at all.