Load Profiles without deleting scores, merge profiles

Having my scores deleted from the machine profile every time I change AdditionalSongsFolders has been a long-standing irritation for me, so this pull request removes that code.
Additionally, this provides functions for merging scores from one profile into another, or into the machine profile, which should help with migrating profiles from older versions.
The user can choose to merge two profiles from the menu on ScreenManageProfiles.

Comment explaining when merging should be used copy pasted from code:
// MergeScoresFromOtherProfile has three intended use cases:
// 1.  Restoring scores to the machine profile that were deleted because the songs were not loaded.
// 2.  Migrating a profile from an older version of Stepmania, and adding its scores to the machine profile.
// 3.  Merging two profiles that were separate together.