Right-click options: Waypoint at mouse cursor position

#### Issue details

This right-click option places the waypoint at the craft's current position or if not connected to a craft then at 0, 0 long lat.

Please could an extra option be added to place the waypoint at the mouse's cursor position (i.e. the position when right clicked)?

This makes more sense than at the craft's position especially if pre-planning a mission while not connected to the craft. It will also speed up adding waypoints dramatically as the waypoint will be placed exactly where one right-clicks.

#### Version
V. 1.3.xx

#### Platform
[ X ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
Plane, Copter

#### Hardware type
Pixhawk, Cube

#### Logs
N/A

Start in Mission Planner's map UI at the existing right-click waypoint option and trace how it chooses the craft position or 0, 0 when disconnected. Done means an additional option places a waypoint at the map cursor's clicked coordinates, including during mission planning without a connected craft.

Written by the indexing model from the issue text.