Add the ability to access beatmap files from the editor

[peppy]

A (relatively) common paradigm for a mapper is to perform operations on the .osu or .osb files contained within a beatmap. This can either be copy/pasting the raw code itself, or drag-dropping the files into a third party program to perform mutation or inspection.

We should give the user a way to access the raw files on demand, likely creating a temporary folder on disk, locking the game interface, and waiting for the user to perform operations, at which point they would press a button in the game and the folder would be cleaned up.

An alternative may be to always follow this paradigm on entering the editor, pulling the files out when entering (as a background operation?) to allow for ongoing edits, and using a FileSystemWatcher to handle any external changes. This implementation is obviously a higher bar in terms of complexity.

[yyny]

We don't necessarily need FileSystemWatcher for synchronization (Although that would be more snappy and less intrusive), we can simply check for file changes whenever the game client regains focus. I would recommend that as a start for anyone looking to implement this.

[frenzibyte]

Whether this needs FileSystemWatcher or not probably depends on how the implementation goes, it's a small detail to even care about currently.

[Joehuu]

Implemented in #28800.