Refactor rows with diacritic signature of IPASkeleton

[taesungh]

Currently, the IPASkeleton class has a diacriticSignature property for tracking the signature of diacritics on each row of the table. This needs to have the same length as the cells matrix and is inherently detached from the table structure. It would be more cohesive to have a single object represent a row of the table along with a property for the diacritic signature. That way, each row holds both the cells and the signature as a unit.

• Refactor the cells matrix to be an array of Row objects which hold the cells in that row along with the diacritic signature
  • It might be possible to extend the built-in Array type to include the custom property