MAVGraph: Add option to choose plot bg from active flight mode

Simple little beautification to make it easier to visualise the
effect of changing flight modes.  Uses the same colour pallet
as MAVProxy's mavflightview.

At the moment this cannot be specified at the same time as
--xaxis as for that, background vregions don't make much sense.
You would have to track a per-marker mode/colour instead, and
while that's possible, it seems overkill at this stage.