When opening a file dialog to set the location of a custom path, use the
corresponding user path as the starting location instead of the current
custom path.

When no custom path was set the dialog would be opened with a blank path
which causes Windows (not sure about other platforms) to open the dialog
at the same location where the last dialog was closed, or at the current
working directory if no previous dialog had been opened.

If a nonempty custom path has been set then it has also set the
corresponding user path, so the behavior in that case is unchanged.

For a few years now, I've been thinking it would be nice to make Dolphin
support reading Wii games in the format they come in when you download
them from the Wii U eShop. The Wii U eShop has some good deals on Wii
games (Metroid Prime Trilogy especially is rather expensive if you try
to buy it physically!), and it's the only place right now where you can
buy Wii games digitally.

Of course, Nintendo being Nintendo, next year they're going to shut down
this only place where you can buy Wii games digitally. I kind of wish I
had implemented this feature earlier so that people would've had ample
time to buy the games they want, but... better late than never, right?

I used MIT-licensed code from the NOD library as a reference when
implementing this. None of the code has been directly copied, but
you may notice that the names of the struct members are very similar.
https://gitlab.axiodl.com/AxioDL/nod/blob/c1635245b881ed0004ff5e616896579ce1b19164/lib/DiscIONFS.cpp