Currently, GameFile returns a generic banner if the file didn't have one
available (either because the file format doesn't support it, or because
it's a Wii file without an associated save).
It makes more sense to handle the lack of banner in the UI layer. The
game list will use the generic missing banner explicitly (no change from before), and the game info window now omits the banner display entirely if the file didn't have one (since it's not useful to display/allow the user to save the "missing banner" banner).
It's not particularily useful to list the platform here,
and these kinds of messages that use words as parameters
are more likely to be mistranslated than the average string.
At first there weren't many enums in Volume.h, but the number has been
growing, and I'm planning to add one more for regions. To not make
Volume.h too large, and to avoid needing to include Volume.h in code
that doesn't use volume objects, I'm moving the enums to a new file.
I'm also turning them into enum classes while I'm at it.
