Add group monitoring mode (v1.0.5)

- Monitor button between callsign and TG selector
- On: reconnects with TG_MONITOR for all switch-list groups,
  status shows 'Мониторинг: TG...', PTT disabled
- Talker display shows callsign and its TG
- Off: back to normal talk mode

app/src/main/res/drawable/ic_headset.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="24dp"
+    android:height="24dp"
+    android:viewportWidth="24"
+    android:viewportHeight="24">
+    <path
+        android:fillColor="#8B949E"
+        android:pathData="M12,1c-4.97,0 -9,4.03 -9,9v7c0,1.66 1.34,3 3,3h3v-8L5,12v-2c0,-3.87 3.13,-7 7,-7s7,3.13 7,7v2h-4v8h3c1.66,0 3,-1.34 3,-3v-7c0,-4.97 -4.03,-9 -9,-9z" />
+</vector>