Improve storeResources file resolution

This patch does three things -
1. Use QUrl::toLocalFile, when testing if the url exists using QFileInfo
2. Before resolving all the url present in the objects. Convert them
from the QVariant into Soprano::Node using the ClassAndPropertyTree,
this way based on the type, the variant is converted to the correct
node. Resolution is a lot simpler ( and correct ) this way.
3. Fix SyncResource blank node handling - When blank nodes are stored
as QUrls they should be stored as "_:identifier" not "identifier"

This fixes testStoreResources_folder ( and strigi indexing )