Add support for DO-SET-HOME

In a flight I planned I intended to set Home to a reasonable point that I know is free of obstacles. I noticed that I couldn't just right-click and "set home" at that point, but I had to drag the existing point all the way over to the new location, requiring a number of zoom-out, drag and zoom-in operations.
It would be handy to be able to just right-click and set home there.
When the vehicle is attached, it asks to "reset home to loaded coordinates", which isn't related at all to coordinates in some flight plan, but the current location of the vehicle. I suggest changing that message accordingly so it makes it more clear.

Start with the map context-menu flow for setting the home point and the attached-vehicle prompt that says "reset home to loaded coordinates." Verify that right-clicking a free map location can set Home directly and that the prompt describes the vehicle's current location rather than flight-plan coordinates.

Written by the indexing model from the issue text.