Allow user configuration of RadioLogger pcap file

Fix for issue #252. It adds a new entry (pcap_file) in RadioLogger configuration in .csc file allowing to specify the pcap file for "Analyzer with PCAP". If the entry is not defined it falls back on the legacy file name (radiolog-<timestamp>.pcap)

Two remarks : 

* I'm not sure if the discard attribute is really needed :
<pre>element.setAttribute("EXPORT", "discard");</pre>
* I had to make an explicit reference to the instance of IEEE802154Analyzer that output the pcap and I'm not really happy with that.

Once this PR is accepted and merged, I'll have a look at the file dialog (still not sure where to put it in the GUI)