Initial PcapNG file format support

[martinling]

This draft PR adds initial support for the PcapNG file format.

The following features are supported:

• Saving captures to a file with:
  • Native endianness
  • A single section header
  • A single interface with the USB_2_0 link type only
  • Nanosecond timestamp resolution
• Loading captures from files with:
  • Any endianness
  • A single section header
  • A single interface with one of the following link types:
    • USB_2_0
    • USB_2_0_HIGH_SPEED
    • USB_2_0_FULL_SPEED
    • USB_2_0_LOW_SPEED
  • Any timestamp resolution

[martinling]

Added support for metadata:

• Storing metadata about the current capture in a CaptureMetadata type.
• Loading and saving metadata in PcapNG files.
• Generating appropriate metadata for Cynthion captures.
• A UI dialog for viewing the capture metadata and editing the comment field.

Some further steps here are blocked on external issues:

1. It's not currently possible to save the comment field in the section header, due to a lifetime issue in the pcap-file crate, which I have submitted a fix for:
   Support non-static lifetimes for PcapNgWriter blocks courvoif/pcap-file#41

2. I'd like the user to be able to select between the generic USB_2_0 link type and the speed-specific e.g. USB_2_0_HIGH_SPEED link types when saving, as applicable to the current capture. This choice applies to both the Pcap and PcapNG formats, so would be best offered as an extra dropdown in the save dialog. This is blocked on a gtk4-rs bug which I've had a fix merged for, but we're still waiting on a new release before we can use it:
   Fix segfault in FileChooserExtManual::add_choice() gtk-rs/gtk4-rs#1834

[martinling]

My gtk4-rs fix is now released in 0.9.2.