Send waypoint range to mav

It would be very useful for environmental mapping to select a waypoint range wich is send to the mav.

An additional button "Send Waypoint Range" wich opens a window where you can select the first and last waypoint to be send. Maybe a dialog which asks for "RTL after last waypoint?"

So its very easy to split the whole mission into small missions for each lipo.

No files or tests are named. Start by locating the existing waypoint mission-sending flow and its mission-planning UI, then determine how a first and last waypoint can be selected and whether RTL-after-last should be supported; done means a selected waypoint range can be sent as a mission to the MAV.

Written by the indexing model from the issue text.