no support for concurrent instances by multiple users on same system

[brainchild0]

If one user has an open instance of YAC, then another user cannot successfully use the application on the same system.

Problems the user is likely to encounter include:

1. The Library application silently refusing to start a new instance.
2. The Library application failing to open a new viewer window, reporting that the file is not found.
3. The Library application starting, but complaining of being unable to start the server.
4. The viewer complaining that the Library application is not responding.
5. A viewer window opening, displaying the name of a file previously opened by an instance owned by another user.

The last behavior is particularly disturbing because it leaks data to a different user.

Generally, an application should not communicate with or detect an instance owned by another user unless sharing data among users is part of the application design.