Store original dimensions results in IV store

* Remove the this.mediaSize custom store object and replace
  with a hash function and the document's IV store.
* Remove this.inputs/this.fieldsets namespaces.
* Calculate this.filename as soon as this.mediaNode is set.
* Rename getMediaSize to getOriginalDimensions

Change-Id: I2030aade5d96555451f6a390d0aa3d44b860841f
Merge "Store original dimensions results in IV store"