[ENHANCEMENT] Chart Editor - Modify chart & metadata difficulties separately

[Lasercar]

Linked Issues

Fixes #5206

Description

Modifies the metadata toolbox and difficulty toolbox to show difficulties that only appear in that data, and allows you to add and remove difficulties from each separately.

Also makes a few other minor changes, such as disabling the remove buttons under some circumstances, and moving the add variation button to the metadata toolbox.

Known Issues: The vertical sliders don't work unless you force them to hide and reappear??? Haxe UI issue?

[!IMPORTANT]
Requires https://github.com/FunkinCrew/funkin.assets/pull/265

Screenshots/Videos

[Hundrec]

Merge conflicts again, love the Chart Editor

[Lasercar]

That's it?

That's the entire merge conflict?

[MightyTheArmiddilo]

At some point during Chart Editor development that metadata window's gonna take up the whole screen-

[Lasercar]

https://github.com/FunkinCrew/Funkin/pull/5215#issuecomment-3602558353

I've already done this, and I've also included an option to delete the difficulty from the toolbox / data you're removing it from, rather than both. I haven't made any changes to account for the new move/clone difficulty commands.

You don't get much control over the contents of a message box, it's either a yes/no, or a yes/no/cancel, so the change might be a bit confusing to users at first, not sure if there's a easy way to fix it. If I could color the text red or something it might help...