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/main/windows/main.js

@@ -206,13 +206,13 @@ function toggleFullScreen (flag) {
 }
 
 function onWindowBlur () {
-  menu.onWindowBlur()
-  tray.onWindowBlur()
+  menu.setWindowFocus(false)
+  tray.setWindowFocus(false)
 }
 
 function onWindowFocus () {
-  menu.onWindowFocus()
-  tray.onWindowFocus()
+  menu.setWindowFocus(true)
+  tray.setWindowFocus(true)
 }
 
 function getIconPath () {