Prompt split editors to save on close

Previously an editor that was split into multiple panes would not
prompt to save correctly when the window was unloading.

This adds a new `windowCloseRequested` option passed through from the
beforeunload handler to the editor so that it can specially handle this
case.

Closes #5257

src/workspace.coffee

@@ -599,8 +599,8 @@ class Workspace extends Model
   saveAll: ->
     @paneContainer.saveAll()
 
-  confirmClose: ->
-    @paneContainer.confirmClose()
+  confirmClose: (options) ->
+    @paneContainer.confirmClose(options)
 
   # Save the active pane item.
   #