Jbrowse JBrowse

Added support for maintaining a selection set of user-selected features (to be used to create annotation(s)).
Currently, you can add features to the selection set by clicking them (clicking again will deselect).
If you drag a selected feature to the annotation track, EACH of the currently selected features creates a
new annotation. (In the future, we'd like to create one multi-span annotation out of all the selected
features.)
Note that while dragging, you only see the drag-ghost of the most recently selected feature. This is
because JQuery doesn't offer direct support for multi-dragging. I investigated various approaches but
didn't find a straightforward solution that would work in Firefox. I left some commented-out attempts
in the code in case they are illuminating.
https://github.com/GMOD/jbrowse/commit/8841f473b7d67a633040607f541f...