UserFileVersionHandler: Api clean up, rename functions

Also rename class from UserFileVersionHandler to VersionUpgrader
userFileVersion -> version
displayUserFileVersion -> backupExtension
update -> upgrade

Change-Id: I2641b6eaee13aa07380a3ccf0e4c13d61f18f920
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>