Edge code/create file with save as after review

Squashed version of reviewed PR: https://github.com/adobe/brackets/pull/4201

Implementing "Create file w/ Save As" user story
https://trello.com/card/3-create-file-w-save-as/4f90a6d98f77505d7940ce88/286

Please approve and merge along with associated Shell changes submitted to brackets-shell as PR # 258 (https://github.com/adobe/brackets-shell/pull/258).