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/torrent-list.js

@@ -44,6 +44,7 @@ module.exports = class TorrentList extends React.Component {
     if (torrentSummary.playStatus) classes.push(torrentSummary.playStatus)
     if (isSelected) classes.push('selected')
     if (!infoHash) classes.push('disabled')
+    if (torrentSummary.torrrentKey) console.error('Missing torrentKey', torrentSummary)
     return (
       <div
         key={torrentSummary.torrentKey}