Cleanup

Change-Id: I701f83da3e4673db12c87e81751c1db5ef77ab91
Refactor to support migrating dictionary configurations

* Extract some dialog specific code from CreateComboAndTreeItems.
* Create an ILayoutConverter interface which the XmlDocConfigureDlg
  and the DictionaryConfigurationMigrator will use.
* Rename CreateComboAndTreeItems to BuildTreeFromLayoutAndParts,
  make it and all its supporting methods static using ILayoutConverter
  parameters to retrieve information from the dialog and migrator.
* Graduate LayoutLevels to a full fledged class rather than an
  internal class of XmlConfigureDlg so it can be used in the migrator.
* clean up some code and add some comments

Change-Id: I4485e3ba70f8b013eca3a8f0c8a6384e4b28088e