shortcuts.js: Consistent exported method naming

Exposed methods whose sole purpose is notify the module of an event
firing, should start with "on".

main/menu.js

@@ -5,6 +5,8 @@ module.exports = {
   onToggleFullScreen,
   onWindowHide,
   onWindowShow,
+
+  // TODO: move these out of menu.js -- they don't belong here
   showOpenSeedFiles,
   showOpenTorrentAddress,
   showOpenTorrentFile,