Don't enforce the menubar icon

I have `Run Terminal.app as background app` enabled and it's not possible to disable the menubar icon. That is understandable as there is no other way to get to `Preferences`.

This is however annoying since the current icon is very ugly in Dark Mode (https://github.com/binaryage/totalterminal/issues/112#issuecomment-80980328).

I think there are better ways to solve this though:
1. Open the `Preferences` if TotalTerminal is started when it's already started.
2. Have a separate helper application called `TotalTerminal Preferences` which opens Preferences.
3. Inject a way to get to the Preferences from the Terminal. Maybe by right-clicking the Terminal window and selecting `Preferences` or something.