gqrx: new package

gqrx: new package

Gqrx is an open source software defined radio (SDR) receiver implemented
using GNU Radio and the Qt GUI toolkit. Currently it works on Linux
and Mac with hardware supported by gr-osmosdr, including Funcube Dongle,
RTL-SDR, Airspy, HackRF, BladeRF, RFSpace, USRP and SoapySDR.

Gqrx can operate as an AM/FM/SSB receiver with audio output or as an
FFT-only instrument. There are also various hooks for interacting
with external application using network sockets.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
[Thomas:
 - remove unneeded dependency from Config.in, inherited from Qt5
 - add entry to DEVELOPERS file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>