LibPartition: Migrate from DeprecatedFile to File

The implemented cloning mechanism should be sound:
- If a PartitionTable is passed a File with
  ShouldCloseFileDescriptor::Yes, then it will keep it alive until the
  PartitionTable is destroyed.
- If a PartitionTable is passed a File with
  ShouldCloseFileDescriptor::No, then the caller has to ensure that the
  file descriptor remains alive.
If the caller is EBRPartitionTable, the same consideration holds.
If the caller is PartitionEditor::PartitionModel, this is satisfied by
keeping an OwnPtr<Core::File> around which is the originally opened
file.

Therefore, we never leak any fds, and never access a Core::File or fd
after destroying it.

Userland/Applications/PartitionEditor/PartitionModel.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <LibCore/File.h>
 #include <LibGUI/Model.h>
 #include <LibPartition/PartitionTable.h>
 
@@ -36,6 +37,7 @@ private:
     PartitionModel() = default;
 
     OwnPtr<Partition::PartitionTable> m_partition_table;
+    OwnPtr<Core::File> m_backing_file;
 };
 
 }