Remove CustomDevice, use editablePartitions() method

[mikee47]

This PR tidies up how device in-memory partition tables are updated.

The Device partitions() method returns a read-only (const) reference to the device's PartitionTable suitable for general application use.
A new editablePartitions() method has been added which provides a non-const version so the table can be updated.
This removes the need for the CustomDevice class, which has been removed.

For example:

part = device->createPartition("archive", Storage::Partition::SubType::Data::fwfs, startOffset, size);

becomes:

part = device->editablePartitions().add("archive", Storage::Partition::SubType::Data::fwfs, startOffset, size);

[slaff]

Rename the method name to use camelCase