Added draft of VelocityFilter

From Matt Nicholson. I did my best to match the UI style. I also added a
screenshot and updated the doc (but did not link the image).
Modded the GUI slightly, and fixed a bug whereby the plug-in would leave hanging notes when filtering out notes with velocity > 0
Merge branch 'rhombuslabs-master'

Modded the GUI a bit, and fixed a bug where filtering notes with velocity > 0 would leave hanging notes.