Attach to Unity Process dialog allows attaching multiple times

[citizenmatt]

It's possible to use the "Attach to Unity Process" dialog to attach to the Unity editor multiple times. This can cause debugging to fail (errors with breakpoints not setting with messages like "No found loaded method"). The debugging list should grey out editors/players that are already attached.

Consider removing editors (or maybe just the current attached editor?) from this list.

Also consider creating temporary run configurations for these processes, so it's more obvious that the debugger is attached. For players, a fuzzy matching on hostname/ip/project name (but not port) could mean that the run configuration can be reused. For editors, reuse based on pid or project name (if we pull it from window title) might be appropriate.