Add option to allow only 1 instance of Nemo

[Dan-cer]

It's confusing when another Nemo window opens even though Nemo already has an open or minimized window. This happens in some cases, for example when I click on a download in the browser.
So I think it would be good to have an option to allow Nemo to open in only 1 instance at a time. Some other file managers have this option.