Beautifier: Small, paranoid local caching in the hope for gaining speed

Avoid that QDir::home() is potentially called twice.

Change-Id: I9fca53f800be74f05d941cf9baa34eecb6c899cb
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: David Schulz <david.schulz@digia.com>