precompiled Win32 binary ExtIO DLL

added airspyhf libs new LNA/AGC/THRESH/ATT features and flashing of frequency correction (ppb) value.
added workaround for a bug in HDSDR 2.76, now allows control of AGC modes from main window.
disabling/hiding HF controls (LNA/AGC/..) at frequencies >= 60 MHz

• new settings require device firmware version >= 1.3
  see section 'Firmware Updates' on https://airspy.com/airspy-hf-plus/
• ExtIO Dialog shows all new features and allows control
• in HDSDR's main window:
  • LNA state is displayed with 'IF+0' or 'IF+6', relevant is the '+0' or '+6'.
    Click/move displayed IF slider with left mouse button to change the LNA value.
  • AGC mode is displayed on same button, in front of the value:
    'IF' signs deactivated AGC / 'Auto Atten'
    'Low' or 'High' signs activated 'AGC with that AGC Threshold
    Click with left/right mouse-button switches to MGC and AGC modes.
  • ATT state is displayed with 'RF' button and slider, when AGC is off.
    This button and slider is automatically hidden when AGC is activated.
• This ExtIO should also be usable with other SDR Software, e.g. Studio1
• This ExtIO is also usable with device firmware versions < 1.3
  In this case the new features are deactivated
• added saving of frequency correction (ppb) into flash, also with older firmware