Assign difficulties of osu! charts based on MSD

[goninty]

Currently, when loading a song from .osu files, the individual difficulties are ordered with no regard for the MSD of each difficulty, and are just assigned in the order the files are loaded (alphabetical based on file name).

As seen below, the BG difficulty is a higher MSD than the EZ difficulty, which appears next.

It can be a little confusing trying to guess which difficulty you would like to select if you were just playing the song in osu!mania, as there they are ordered by star rating. I'm aware that ordering by MSD may order things differently than by star rating in osu!, but it'd be better than leaving them ordered how they are now.

It would be a nice enhancement if the "stepmania-based" difficulties (BG, EZ, NM, etc) were assigned relative to the overall MSD value of each difficulty - at least for charts read from .osu files.

I'm aware that sorting based on the MSDs of the individual difficulties is in the works as mentioned in #443, but I'm not sure if that would include sorting .osu files that way by default - even when sorting based on MSD hasn't been explicitly enabled by the user.

Maybe this change can be considered when the feature mentioned above has been implemented.

If this change feels unnecessary / has already been considered / is already in the works, then feel free to insta-close this issue. Cheers!

[poco0317]

To keep the loading of songs from disk optimized and neat, this can only be done if we decide to sort all song difficulties by MSD, which is not preferred due to things such as calc changes possibly reordering the difficulty of files.