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
Before, the app made itself the default torrent file handler automatically, pissing off some of our users. Now, it's not by default, and you can change it in the prefs.
Less code for us to maintain.
This also gives us free internationalization in a future Electron
version (they'll set the label dynamically based on the 'role')
One slight regression with this change, but it will be fixed in a
future Electron once this PR is merged:
https://github.com/electron/electron/pull/6600
