Add Util.openFile() and fix more URL/file opening bugs.

 - The incorrect path was passed when browsing plugins (uploaders instead of plugins/uploaders).
 - The Desktop class was used directly in the HistoryPanel and UploaderPanel classes. I've added a Util.openFile() method which is called instead, and also supports platforms without Desktop support (not tested on Mac/Windows, but I suspect they both support Desktop anyway).