Qspectrumanalyzer

- *hackrf_sweep* backend fixes and rate limiting (thanks to michaellass)
- better Python 3.4 compatibility
- many Windows fixes
- installer for Windows is finally available
- allow setting gain with more precision
- *soapy_power* dependency is now optional (it is still required in setup.py, because it is default backend, but QSpactrumAnalyzer will now actually run even without it)

- allow setting filter bandwidth and LNB LO frequency (negative for upconverters, positive for downconverters)
- show detected SoapySDR devices and info about selected device in *Settings* (works only for `soapy_power` backend)
- some improvements in GUI

- switch from PyQt4 to Qt.py, QSpectrumAnalyzer now supports both PyQt4, PyQt5, PySide and PySide2 (PyQt5 is used by default)
- fix pipe used for communication with soapy_power process on Windows
- allow backend executable in *Settings* to be composed of multiple words (so e.g. `python /path/to/soapy_power.py` can by used)

- use pipe for communication with soapy_power process, to avoid reading garbage from stdout (some SoapySDR drivers like LimeSDR always outputs something to stdout)

- new universal [soapy_power](https://github.com/xmikos/soapy_power) backend
- rx_power backend
- hackrf_sweep backend (wideband spectrum monitoring with sweep rate of 8 GHz/s)
- allow setting additional backends command line parameters
- show help for backends in settings