Fix screen stacking bug

You can no longer open a whole stack of Prefs windows, or Create Torrent windows

Simplifies and fixes behavior when dropping files onto the app or the dock icon. Before, you could use drag-drop to create stacks of Create Torrent windows. Now, you can only create torrents from the home screen.

Fixes #665

src/renderer/views/create-torrent-error-page.js

@@ -16,7 +16,7 @@ module.exports = class CreateTorrentErrorPage extends React.Component {
           </p>
         </p>
         <p className='float-right'>
-          <button className='button-flat light' onClick={dispatcher('back')}>
+          <button className='button-flat light' onClick={dispatcher('cancel')}>
             Cancel
           </button>
         </p>