Add initial panel API. 

It can only add things to the left right now.
Add panel containers
Panel containers have the orientation
Integrate the pane containers into the workspace
Render proper atom-panels as children of containers
Panel container probably not public
orientation -> location
Rename custom element so as not to clash
Remove location junk from panel creation
Add matcher toHide()
Add hide/show ability for panels
Set the heights of atom panels for location left and right.
Add priority system to the panels
Fix spec
Doc Panel class
:memo: Doc the new panel methods
Deprecate workspace pane methods
:memo: Move panels API for the sake of the docs
Call the attach hooks after adding a view to a panel.
:lipstick: