SettingsAccessor: Refactor helper functions for merge/track sticky

Instead of a template use a abstract base class.

Change-Id: I92675bf1c19ef60bec22836eda56240c2f2e4c80
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Rename Version$Handler to UserFileVersion$Upgrader

That name is cleaner.

Change-Id: Idafc6d01b79f1748884a77fe06d407a76fa83026
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>