Support for ~, %username% and %homedrive% in backup paths

Added support for ~, %username% and %homedrive% in backup paths, for the LoadBackupDirectory, SaveBackupDirectory and PreviousBackupDirectory paths. 

Made a PathHelpers class, so if there is any other paths, or future paths that need this, it's reusable and plus it looks neater.

Hope I did good on my first commit :)