first stab at ft8 decoding: chop up audio, call jt9 binary to decode

htdocs/index.html

@@ -109,6 +109,7 @@
                         <select id="openwebrx-secondary-demod-listbox" onchange="secondary_demod_listbox_changed();">
                             <option value="none"></option>
                             <option value="bpsk31">BPSK31</option>
+                            <option value="ft8">FT8</option>
                         </select>
                     </div>
                     <div class="openwebrx-panel-line">

htdocs/openwebrx.js

@@ -2633,6 +2633,7 @@ function demodulator_digital_replace(subtype)
     {
     case "bpsk31":
     case "rtty":
+    case "ft8":
         secondary_demod_start(subtype);
         demodulator_analog_replace('usb', true);
         demodulator_buttons_update();
@@ -2809,6 +2810,9 @@ function secondary_demod_listbox_changed()
         case "rtty":
             demodulator_digital_replace('rtty');
             break;
+        case "ft8":
+            demodulator_digital_replace('ft8');
+            break;
     }
 }