Add classes for mounting FAT16/32 without help from OS

Minimal implementation for reading/writing files in the root
directory of a FAT16/FAT32 file system.

Can read/write from raw disk devices, and no longer relies on
operating system support for mounting the file system.

Currently assumes Imager will always be run on 'little endian'
architectures such as Intel and ARM (at least under Linux).
If there is a use-case for big-endian (anybody still using Sparc?)
this may be revisited later.

src/devicewrapper.h

@@ -0,0 +1,50 @@
+#ifndef DEVICEWRAPPER_H
+#define DEVICEWRAPPER_H
+
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ * Copyright (C) 2022 Raspberry Pi Ltd
+ */
+
+#include <QObject>
+#include <QMap>
+#include <QFile>
+
+class DeviceWrapperBlockCacheEntry;
+class DeviceWrapperFatPartition;
+
+#ifdef Q_OS_WIN
+#include "windows/winfile.h"
+typedef WinFile DeviceWrapperFile;
+#elif defined(Q_OS_DARWIN)
+#include "mac/macfile.h"
+typedef MacFile DeviceWrapperFile;
+#else
+typedef QFile DeviceWrapperFile;
+#endif
+
+
+class DeviceWrapper : public QObject
+{
+    Q_OBJECT
+public:
+    explicit DeviceWrapper(DeviceWrapperFile *file, QObject *parent = nullptr);
+    virtual ~DeviceWrapper();
+    void sync();
+    void pwrite(const char *buf, quint64 size, quint64 offset);
+    void pread(char *buf, quint64 size, quint64 offset);
+    DeviceWrapperFatPartition *fatPartition(int nr);
+
+protected:
+    bool _dirty;
+    QMap<quint64,DeviceWrapperBlockCacheEntry *> _blockcache;
+    DeviceWrapperFile *_file;
+
+    void _readIntoBlockCacheIfNeeded(quint64 offset, quint64 size);
+    void _seekToBlock(quint64 blockNr);
+
+signals:
+
+};
+
+#endif // DEVICEWRAPPER_H