SpiceAgent: Handle file transfer requests properly :^)

Now, we write the data recieved to a file when the user drags a file
onto the Spice Viewer window. Once complete, the FileExplorer will open
with the copied file highlighted.

Userland/Services/SpiceAgent/SpiceAgent.h

@@ -8,6 +8,7 @@
 #pragma once
 
 #include "ChunkHeader.h"
+#include "FileTransferOperation.h"
 #include "Message.h"
 #include "MessageHeader.h"
 #include <AK/MemoryStream.h>
@@ -22,6 +23,9 @@ namespace SpiceAgent {
 // If the buffer's length is equal to this, then the next data recieved will be more data from the same buffer.
 constexpr u32 message_buffer_threshold = 2048;
 
+// The maximum amount of data that can be received in one file transfer message
+constexpr u32 file_transfer_buffer_threshold = 65536;
+
 class SpiceAgent {
 public:
     static ErrorOr<NonnullOwnPtr<SpiceAgent>> create(StringView device_path);
@@ -61,6 +65,7 @@ public:
 private:
     NonnullOwnPtr<Core::File> m_spice_device;
     Vector<Capability> m_capabilities;
+    HashMap<u32, NonnullRefPtr<FileTransferOperation>> m_file_transfer_operations;
 
     RefPtr<Core::Notifier> m_notifier;