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.