ISOFile and GameFile were using IsWiiDisc() and IsWadFile() to set
an enum value. The volume might as well return an enum directly.

I increased the Qt CACHE_REVISION because m_platform now is saved as u32
instead of int, but increasing the wx CACHE_REVISION is not necessary.

Filesystem: Return file list reference instead of modifying argument

This is good for performance when FileMontior is selected
but the log level only is set to notice or warning.
The ability to do this wasn't available until recently:
1ed41672f50c9cfe7aa4ab5aa3b73f7e41029124

Don't read from disk when checking volume type

Should be faster than relying on the OS to cache the magic words.
Also gets rid of the odd recursive call in VolumeDirectory.