Detached, centered overlays

First steps for issue/feature #964. This provides basic alignment and styling for creating centered overlays.

To create a centered overlay, use the `centered` class instead of the `from-top` class, and call `centerOverlay()` after attaching to the workspace:
````coffeescript
initialize: ->
  ...  
  @addClass('command-palette overlay centered')
  atom.workspaceView.command 'command-palette:toggle', => @toggle()

...

attach: ->
  ...
  atom.workspaceView.append(this)
  @centerOverlay()
  @focusFilterEditor()
````