Improve area mode

[dabreegster]

• "Backfill" or import an existing polygon produced in another tool, inferring waypoints
• Allow freehand points. Maybe requires first toggling existing waypoints between snapped & free
• Allow deleting the first=last waypoint
• Sometimes a waypoint winds up looking like a linear route that doubles back on itself; the resulting polygon is a bit broken.
• After drawing a closed polygon, I've sometimes clicked and managed to incorrectly insert a new waypoint at the very end, creating a linestring that escapes the polygon