FF atom dragging: disable dragstart events on document

[#45233971]

Another hack to make dragging atoms work better in FF.

The second of two closely-space clicks generates a dragstart
event in FF which causes atom dragging to not work until
after a subsequent click in the model not on an atom.

I'm not sure why this needs to be a dragstart listener
on the document and not for example just on the atoms
or SVG group container they are a part of.
Atom dragging: disable display of user selection

[#45233971]

One of the unfortunate results of the work to get atom
dragging working when extending outside of an iframe
boundary is that mousedown events have need to bubble
up to the window level.

This causes an unfortunate effect of making selectable text
and elements highlight to show their selection.

This will need to be adapted when and if we need to support
selection in the model container -- for example entering
text for a label. This is likely to come up when we start
work on WYSIWYG model editing.