Allow PanelElements to be instantiated with markup

src/panel-element.coffee

@@ -1,5 +1,6 @@
 {CompositeDisposable} = require 'event-kit'
 {callAttachHooks} = require './space-pen-extensions'
+Panel = require './panel'
 
 class PanelElement extends HTMLElement
   createdCallback: ->
@@ -13,14 +14,15 @@ class PanelElement extends HTMLElement
     @subscriptions.add @model.onDidDestroy(@destroyed.bind(this))
     this
 
-  getModel: -> @model
+  getModel: ->
+    @model or= new Panel({})
 
   getItemView: ->
-    atom.views.getView(@model.getItem())
+    atom.views.getView(@getModel().getItem())
 
   attachedCallback: ->
     callAttachHooks(@getItemView()) # for backward compatibility with SpacePen views
-    @visibleChanged(@model.isVisible())
+    @visibleChanged(@getModel().isVisible())
 
   visibleChanged: (visible) ->
     if visible