Add ghost atoms to elements list in the Draw Tool's pane or to Build menu

[matterhorn103]

As far as I know, the only current way to create a ghost/dummy atom is by creating one of a normal element, then changing the element to Xx in either the coordinate editor or Atom Properties.

We will document this of course as part of the 2.0 docs, but it's still not discoverable or obvious.

I suggest adding it to the list of elements that can be selected from when using the Draw Tool; alternatively, it could be a menu entry under Build, under Insert presumably.