Rename PaneGrid to PaneAxis

PaneGrid is a superclass of PaneRow and PaneColumn. These are both
a type of axis for the pane layout system.

src/app/pane-axis.coffee

@@ -2,7 +2,7 @@ $ = require 'jquery'
 {View} = require 'space-pen'
 
 module.exports =
-class PaneGrid extends View
+class PaneAxis extends View
   @deserialize: ({children}) ->
     childViews = children.map (child) -> deserialize(child)
     new this(childViews)

src/app/pane-column.coffee

@@ -1,9 +1,9 @@
 $ = require 'jquery'
 _ = require 'underscore'
-PaneGrid = require 'pane-grid'
+PaneAxis = require 'pane-axis'
 
 module.exports =
-class PaneColumn extends PaneGrid
+class PaneColumn extends PaneAxis
   @content: ->
     @div class: 'column'
 

src/app/pane-row.coffee

@@ -1,9 +1,9 @@
 $ = require 'jquery'
 _ = require 'underscore'
-PaneGrid = require 'pane-grid'
+PaneAxis = require 'pane-axis'
 
 module.exports =
-class PaneRow extends PaneGrid
+class PaneRow extends PaneAxis
   @content: ->
     @div class: 'row'