`bindiff --ui` should bring existing UI to foreground

[cblichmann]

Steps to reproduce the problem:

1. Run bindiff --ui
2. Run bindiff --ui again

What is the expected behavior?

One instance of the UI should open and the second invocation should bring it into foreground.

What went wrong?

The second BinDiff UI exits, because it cannot listen on its socket server port.

What version of the product are you using? On what operating system?

BinDiff 7, any OS

Any other comments?

Likely implementation not in core, but in UI itself.