REL-2132

Added icon and re-added button to panel.
If they click on this button when the guide star is already active it will deactivate it (leaving no active guide star for the guider).  In other words, the tooltip is exactly the opposite of what will happen in that case.

Ideally we'd change the tooltip text accordingly to something like "deactivate guide star".  At this point maybe we should just make it a weaselly catch-all like "Set/unset as active guide star".  I think since nobody actually uses this button this is probably fine.