Language manager should handle nested blocks when importing or exporting xliff

Description

In the following scenario, the Title field from the PageHeaderBlock would appear in the exported data, but not the AltText or Caption from the ImageBlock that it was using as a property.

Page :: GenericPage

• Block :: PageHeaderBlock
  • String :: Title
  • Block :: ImageBlock
    • String :: AltText
    • String :: Caption
• ContentArea :: Content