Refactor DSi_NAND (#1844)

* Refactor diskio's contents

- Change ff_disk_read_cb/write_cb into a std::function instead of a raw pointer
- Add const specifiers as needed

* Refactor DSi_NAND to manage the file system's mounted lifetime with RAII

* Split NANDMount into NANDMount and NANDImage

- NANDImage is used for information about the NAND that doesn't require decryption or filesystem access
- NANDMount is used to actually access the file system
- Both classes manage their respective resources (the NAND file handle and the NAND's mount) with RAII
- Also split the file loading into another function that I will remove in a later PR

* Make NANDMount immovable

* Remove NAND-loading code that I had sectioned off into a function

- Incomplete copypasta
- I must have gotten distracted

* Tidy up NANDImage's initialization

- Don't unmount the disk image if the constructor fails (that's NANDMount's job now)
- Only assign CurFile if the constructor succeeds

* Add some const-correctness

* Move DSi NAND initialization to the frontend

- The NANDImage is now installed via a unique_ptr in DSi

* Remove Platform::DSi_NANDPath

- Not Config::DSiNANDPath; that can still be configured as usual
- The core no longer needs to care

src/frontend/qt_sdl/ROMManager.h

@@ -22,6 +22,7 @@
 #include "types.h"
 #include "SaveManager.h"
 #include "AREngine.h"
+#include "DSi_NAND.h"
 
 #include <string>
 #include <memory>
@@ -40,6 +41,7 @@ bool LoadBIOS();
 void ClearBackupState();
 
 bool InstallFirmware();
+bool InstallNAND(const u8* es_keyY);
 bool LoadROM(QStringList filepath, bool reset);
 void EjectCart();
 bool CartInserted();