Clear selection when clicking on the adder button.

As discussed in https://github.com/hypothesis/annotator/pull/78.

Fixes https://github.com/hypothesis/h/issues/1598.

h/static/scripts/guest.coffee

@@ -354,6 +354,7 @@ class Annotator.Guest extends Annotator
     event.stopPropagation()
     @adder.hide()
     annotation = this.setupAnnotation(this.createAnnotation())
+    Annotator.util.getGlobal().getSelection().removeAllRanges()
     this.showEditor(annotation)
 
   onSetTool: (name) ->