Implement importing multiple saves from an export folder

[JoshuaVandaele]

Implements https://bugs.dolphin-emu.org/issues/12887

This is my first PR for a feature, so I'd really appreciate feedback on what to improve and/or do differently!

[dreamsyntax]

Don't consider this a blocker, just my opinion:
Is there a way we could combine the Import Wii Save... option with this?
Such that you could pick a folder or a save, or multi select some saves?
If this is out of scope, don't worry about it, just the first thought I had seeing this.

[JoshuaVandaele]

I had looked into doing this at first, however neither Windows, Gnome, or OSX support selecting files and/or a directory at the same time in their native file dialog... So we'd have to make a custom file and/or folder selection widget, which I feel is a bit ugly and out of scope for this PR.

[AdmiralCurtiss]

Haven't looked at the code in detail but this seems cool, definitely a usability improvement.

[jordan-woyak]

I think it's usually preferred to have buttons with verbs like "Overwrite" and "Skip" instead of "Yes" and "No".

But if that's majorly inconvenient, I'd at least have an full empty line before the final line saying "Overwrite existing save?".

[JoshuaVandaele]

I think it's usually preferred to have buttons with verbs like "Overwrite" and "Skip" instead of "Yes" and "No".

But if that's majorly inconvenient, I'd at least have an full empty line before the final line saying "Overwrite existing save?".

I used the same text from the original "Import Wii Save" feature as much as possible, and this is one of those instances. I think this should be changed in a separate PR and/or commit. (Would it be fine to change within this PR in a separate commit?)