Duplicate file feature in project tree

https://trello.com/c/AUMtxj7D/754-create-file-w-duplicate
https://github.com/adobe/brackets/issues/1921

* Added a contextmenu option for duplicating a file/folder.
* Added a parameter at the createNewItem function in the projectManager to be able to choose if a new file should be empty or copied. (Might implement a duplicateItem function for that?).
* Folder copy does work, but the content of the folder isn't copied yet.

Question is should this be an extension or a core feature of Brackets?
Already discussed this with a few people on the #brackets channel, but I leave it up to the reviewers as I'm not sure what's the best choice.

Please be positevly harsh on me :) There is much to learn and I appreciate any from of comment! 

Did test this on Windows 8.1. Not on linux/mac yet.