Reorganise source code

[martinling]

Apart from the backend directory, the Packetry source code is currently one big bucket of code. Some of the files, particularly capture.rs, are huge. There is no index of what lives where. This makes it difficult for new contributors to find their way around and get started.

This PR tries to make it a bit easier for people to find their way around the source code. The main changes are:

• A top-level comment is added to each file to provide an overview of its contents.
• All UI-related code is moved into the ui module (tree_list_model, model, row_data, item_widget, record_ui and test_replay).
• The database primitives are moved into a new database module (stream, data_stream, index_stream and compact_index).
• The tree item API is split out of capture into a new item module (ItemSource, TrafficItem, DeviceItem etc) .
• All general utility code is moved into the util module.

This leaves a much more manageable set of files at the top level, including:

• Standalone programs (main.rs, cli.rs and fuzzer.rs)
• The build script and version info (build.rs and version.rs)
• The key modules that contain most of the core functionality (usb.rs, decoder.rs, capture.rs, item.rs, pcap.rs)

There is further work that could be done to break some of the larger files down further and add commentary, but I think this makes for a reasonable start.

There are no functional changes. This PR is purely reorganisation.