Refactored the code to allow generating scan data files
independently from the add-in database update process.
The new AddinFolderVisitor class can be used to visit
all the add-in folders. Split the functionality previously
in AddinScanner in two classes. AddinRegistryUpdater is
now in charge of locating the addins. AddinScanner now
only implements the actual add-in scanning code, and the
logic to setup a custom assembly resolver has moved there.
