EXP-860 FIX -- Display NEW for items added to inbox

* Added createFolderView, createFolderViewFolder and createFolderViewItem virtuals
  to LLInventoryPanel so these can be overridden by a derived class for custom display.
* Added new LLInboxInventoryPanel class that overrides createFolderViewFolder with a
  LLInboxFolderViewFolder for custom display with NEW tags.
* LLInboxFolderViewFolder NEW tag is tied to the mFresh variable that currently has
  no logic attached to it, meaning it remains off all the time.

Reviewed by Richard.