Add support for layered hit sounds skin option
Implement rate adjustment for storyboard samples
Standardise all tournament tests to use placeholder data
Increase coordinate parsing limits
Now asserting instead of an if-statement, change cast from OsuStorage to MigratableStorage and make internal virtual properties protected.
Add text explaining a second copy will be made on importing beatmaps
Add classic "Welcome" intro
This is (also) to address the review from bdach about StorageManager initialising a default value that gets overwritten upon migration anyway.
Also changes the class accessor from internal to public