Implement fallback decoder registration

After the preparatory introduction of LineBufferedReader, it is now
possible to introduce registration of fallback decoders that won't drop
input supplied in the first line of the file.

A fallback decoder is used when the magic in the first line of the file
does not match any of the other known decoders. In such a case,
the fallback decoder is constructed and provided a LineBufferedReader
instance. The process of matching magic only peeks the first non-empty
line, so it is available for re-reading in Decode() using ReadLine().

There can be only one fallback decoder per type; a second attempt of
registering a fallback will result in an exception to avoid bugs.

To address the issue of parsing failing on badly or non-headered files,
set the legacy decoders for Beatmaps and Storyboards as the fallbacks.

Due to non-trivial logic, several new, passing unit tests with possible
edge cases also included.

osu.Game/IO/LineBufferedReader.cs

@@ -25,6 +25,7 @@ namespace osu.Game.IO
 
         /// <summary>
         /// Reads the next line from the stream without consuming it.
+        /// Subsequent calls to <see cref="PeekLine"/> without a <see cref="ReadLine"/> will return the same string.
         /// </summary>
         public string PeekLine()
         {
@@ -39,6 +40,7 @@ namespace osu.Game.IO
 
         /// <summary>
         /// Reads the next line from the stream and consumes it.
+        /// If a line was peeked, that same line will then be consumed and returned.
         /// </summary>
         public string ReadLine() => lineBuffer.Count > 0 ? lineBuffer.Dequeue() : streamReader.ReadLine();