Include "Close Unmodified Documents" option on Editor tab menu

Include a new option on contextual Editor tab menu: Close Unmodified Documents.

It can solve "tabs hell" problem easily.

In the past, theses two plugins do the same work:

https://github.com/Chris2011/nb-git-close-unmodified-documents

and

https://github.com/oosterholt/nb-close-unmodified-tabs 

It is very trivial to implement it on https://github.com/apache/netbeans/blob/master/platform/core.io.ui/src/org/netbeans/core/io/ui/IOWindow.java

I'm opening this issue just to discuss this feature before start to do a pull request.