This ports the Wii filesystem root, Wii SD card path and dump path
settings to the new config system (OnionConfig).
My initial plan was to wait until DolphinWX was removed before porting
most of the Main (Core, DSP, General) settings to onion config, but
I've decided to submit a small part of those changes to fix
[issue 10566](https://bugs.dolphin-emu.org/issues/10566).
Removes the need to manually set the FileUtil path in the UI frontends
and gets rid of some more members that don't really belong in SConfig.
Also fixes a bug which would cause the dump path not to get created
after change.
c99ac40700