** Fix for #431 **

-> See https://bugzilla.mozilla.org/show_bug.cgi?id=559506
-> Removed the javascript:void(0) href for anchor elements in the multiselect dropdown
-> Added a tabindex = 0 to keep the anchors focus-able
-> Added code to remove any text selection when using shift+click
-> Handles the checkbox selection manually when done using shift+click on its label