[ProbeRequestFilter] Remove unneeded define
[MacAddress] Fix toString()

QByteArray.operator[] returns a char, but we need it to be interpreted
as an unsigned char by QString::number(). Otherwise it'll consider any
char with the MSB set to be negative, leading to unwanted results.
[build] Build with debug information

It turns out we occassionally write bugs. Generate debugging information
so we have a fighting chance to fix those bugs.
[MacAddress] Get manufacturer based on the MAC address

We can now figure out the manufacturer of a device based on the mac
address.

This is a reasonably simplistic implementation, and it won't handle
cases where the manufacturer prefix isn't exectly three bytes.