Introduce efficient member functions for deleting waypoints (#1151)

Introduce efficient member functions for deleting waypoints (#1151)

* Introduce efficient member functions for deleting waypoints

New functions with complexity O(n) for deleting pre-marked
waypoints are:
  WaypointList::del_wpts
  RouteList::del_wpts
  del_wpts
  route_del_wpts
  track_del_wpts

Use those functions instead of the inefficient versions
WaypointList::waypt_del
RouteList::del_wpt
waypt_del
route_del_wpt
track_del_wpt.
When these functions are using while looping over a waypoint list
the overall complexity is O(n^2).  This is because these functions
themselves amortize to O(n).

* review nits.

* tweak deletes

* use dedicated wpt deletion flag.