Improve send/receive dialogs

[andynoack]

Add new features:

• Implementation: Show/Hide GUI elements depending on device has method e.g. if hasattr(self.device, set_if_gain): if_slider.show()
• collapsable gain spinboxes for detailed gain config (see HackRF Gain sliders & LNA button #136). Test useability of slider and spinbox (slider left of spinbox)
• consider supported gains for device e.g. RTL-SDR: get_tuner_gains and write max. value behind gain, e.g. Gain: (max. 400) or behind textfield ("/400") depending on the space in the dialog (see RTL-SDR Low Signal Strength on Record Signal #177)
• direct sampling support for RTL-SDR
• ppm support for RTL-SDR (rtl-sdr ppm correction #189)
• make freq correction configurable (RTL-SDR)
• consider Add support for USRP B200/B210 #133 (comment) for USRPs
• AirSpy accepts only two values for sample rate: 2.5MSPS and 10MSPS. (see Sample rate limitations #166)
• consider available samples rates for devices
• show remaining RAM in receive dialog
• make the current 60% RAM buffer configurable in options
• also write success messages e.g. "successfully set sample rate to ..." to log in textbox, not only errors

[dkozel]

A useful set of USRP configuration settings for a single channel would include:

• Subdevice Specification (subdev_spec)
• TX/RX antenna
• TX/RX bandwidth

The valid values for all these can be queried with the multi_usrp API.
http://files.ettus.com/manual/classuhd_1_1usrp_1_1multi__usrp.html