Feature: Multi-Root Workspaces

[fanantoxa]

Sublimetext and vscode implements Workspace / Project
documentation here: http://docs.sublimetext.info/en/latest/file_management/projects.html
https://code.visualstudio.com/docs/editor/multi-root-workspaces

This feature allow to work with many folders at the same time and hold special config for it.
For example you can have back-end, front-end and worker as independent applications but feature might require to make changes in all of them

This feature might too ambitious for Pre-Alpha.
Minimal implementation might be to allow add multiple folder to file three

[bryphe]

Related to #640 #639 and #440

[bryphe]

I'll use this to track implementing multi-root workspaces / integrating with the Code extension host.