DolphinQt: Show a warning when launching an NKit disc image

It is my opinion that nobody should use NKit disc images without
being aware of the drawbacks of them. Since it seems like almost
nobody who is using NKit disc images knows what NKit is (hmm, now
how could that have happened...?), I am adding a warning to Dolphin
so that you can't run NKit disc images without finding out about the
drawbacks. In case someone really does want to use NKit disc images,
the warning has a "Don't show this again" option. Unfortunately, I
can't retroactively add the warning where it's most needed:
in Dolphin 5.0, which does not support Wii NKit disc images.

Source/Core/DiscIO/VolumeDisc.cpp

@@ -84,4 +84,10 @@ std::optional<u8> VolumeDisc::GetDiscNumber(const Partition& partition) const
   return ReadSwapped<u8>(6, partition);
 }
 
+bool VolumeDisc::IsNKit() const
+{
+  constexpr u32 NKIT_MAGIC = 0x4E4B4954;  // "NKIT"
+  return ReadSwapped<u32>(0x200, PARTITION_NONE) == NKIT_MAGIC;
+}
+
 }  // namespace DiscIO