Use flexbox to arrange panes

src/pane-container.coffee

@@ -127,14 +127,6 @@ class PaneContainer extends View
       return pane if view?
     null
 
-  adjustPaneDimensions: ->
-    if root = @getRoot()
-      root.css(width: '100%', height: '100%', top: 0, left: 0)
-      root.adjustDimensions()
-
   removeEmptyPanes: ->
     for pane in @getPanes() when pane.getItems().length == 0
       pane.remove()
-
-  afterAttach: ->
-    @adjustPaneDimensions()