Persistent inter-app glyph selection

[alancleary]

Currently multiple instances of GCV (and other applications) interact via select and deselect events that are broadcast during mouseover and mouseout events, respectively. It would be nice if a select event could persist after the corresponding mouseout event. Furthermore, it would be nice if this persistence could be used to select multiple, disparate elements.

For now, implement this persistent selection on the gene glyphs in the micro-synteny viewer. Specifically, broadcast a selection on the mouseclick event, but only after clearing the previous selection with a deselectall event. The deselectall event can be implemented as a deselect event that has no targets specified.