[Packet] Keep data in a QByteArray instead of managing it ourselves.

It's a bit silly not to use the class Qt provides for pretty much
exactly this use case.
[ProbeRequestFilter] Parse the radiotap header

We have to include our own definition of the radiotap header because
unfortunately there doesn't appear to be a package with the definition.

Rather than messing around with unpackaged libraries it's easier to just
include the header. We're only really interested in the length anyway,
so we can parse the rest of the captured data.
[ProbeRequestFilter] Chop off the radiotap header

Add a 'pull()' method to the Packet class. It removes the first `len`
bytes from the packet, i.e. a header of `len` bytes.