liveview: Use argparser for argument parser

Also add --interval option
liveview: Move class definitions out of if __name__
liveview: Whitespace fix
liveview: Conduct redraws from a separate thread

In principle this should improve interactive performance,
unfortunately this doesn't seem to work at the moment
liveview: Avoid double-scheduling redraws
liveview: Fix type of --interval
liveview: Don't reschedule update_plot

This caused redraws to accumulate, significantly and progressively
degrading performance