Restrict client to one running instance

[lyarenei]

Idea from #6
Should be quite easy on Unix, I am not sure about Windows.

[starek4]

We should label that with low priority if you are ok with it. We need to discuss how will the user use our app (if it will be as GUI, as service, etc.) and so on. Now, we certainly have to focus on getting the code to better condition and write unit tests. Then we can do these things, but it is very good that we have them in this backlog.

[lyarenei]

No problem, we can definitely introduce priorities. Can you please make low/medium/high labels and assign them to other issues? Thank you.

[starek4]

Yes, I did that.

[starek4]

05ec95f