Switch to direct use of librtlsdr and PulseAudio

Substantial rework to replace the spawning of rtl_fm and aplay with direct
usage of librtlsdr and PulseAudio in a multi-threaded model. This is required
due to changes in AGL application execution that prevent spawned processes from
exiting, resulting in the plugin hanging on frequency changes or stopping.

The rework has been accomplished by refactoring the source of rtl_fm.c into a
reusable form and connecting it to the RtlFmRadioTunerControl class used to
implement the functionality exposed by the RtlFmRadioPlugin class. The idea for
reusing the source code in rtl_fm.c in this way is inspired by the older
qml_radio_plugin codebase, but a new refactor of rtl_fm.c was done to keep more
of its filtering functionality and ensure behavior consistent with the previous
implementation. The files radio_output.{h,cpp} are adapted from qml_radio_plugin
with some additional modifications.

Other changes include:
- The files in the convenience subdirectory have been copied from the librtldr
  source tree to reduce the effort of importing rtl_fm.c.
- The COPYING file containing the GPL license has been copied from the
  librtlsdr source tree to accompany rtl_fm.c and the convenience/* files.
- The recently added AM band support has been removed as the USB DVB adapters
  are incapable of receiving AM without significant tweaking, and a single
  adapter would be unable to do both AM and FM at the same time. The plugin
  now explicitly reports that it only supports FM.
- The list of known stations to act as an ersatz seeking implementation has
  been removed, as the updated higher-level QML application no longer exposes
  seeking. Adding this functionality back in would be straightforward if it
  becomes required again. There is also some code in rtl_fm.c that could possibly
  be adapted into a proper signal strength detection scheme in the future if
  that is desired.
- A Qt QSettings file is used to store the FM band plan information to
  allow using the specific frequency ranges for North America versus Japan.
  The band plan can be changed by modifying the "fmbandplan" entry in the
  QSettings .conf file to either "US" or "JP". The location of the .conf file
  can be one of:

  $HOME/.config/AGL/qtmultimedia-rtlfm-radio-plugin.conf
  $HOME/.config/AGL.conf
  /etc/xdg/AGL/qtmultimedia-rtlfm-radio-plugin.conf
  /etc/xdg/AGL.conf

Note that some debugging output has been left in place in the start and stop
methods to facilitate debugging of the higher-level QML application.  They will
be removed once that is complete.

Change-Id: I1d92c74eb24b24cb5416dd531b599645d1287295
Signed-off-by: Scott Murray <scott.murray@konsulko.com>

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