Enable Copy/Paste for keyboard users

[prushforth]

Since we enable mouse / pointer / touch users to copy a location, and in the future, pasted it as a feature, we must ensure that the same functionality is available to keyboard / screenreader users.

One idea:

In the mapml-extension, create the option to always draw a reticle / crosshair on any map at its center. This will allow the keyboard user to accurately point at a location. Once they are pointing at a location, they can open the context menu on the map centre, at the crosshair, via Shift+F10, and by selecting Copy > Location, get a copy of the map centre location, which they can then Paste via the context menu as a new feature.