Fix RTLSDR E4000 gain and bandwidth settings and add RTLSDRBlog V4 support

[srcejon]

I noticed two problems with Nooelec RTLSDR with E4000 tuner:

• Gain setting was not working. After adjusting the gain, it would re-adjust itself as though AGC had been reenabled. It appears we need to call rtlsdr_set_tuner_gain_mode before each call to rtlsdr_set_tuner_gain.
• Adjusting RFBW would result in a bizarre frequency response.

Also, added a change to the GUI that displays the tuner type and disables Ofs checkbox if not supported for the given tuner.

[srcejon]

Also added support for RTLSDRBlog V4, by updating to latest RTLSDR driver and setting low high frequency limit to 0kHz to support its integrated HF upsampler. Implements #1783