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/layout/activity_main.xml

@@ -84,6 +84,15 @@
                     android:ellipsize="end"
                     android:text="@string/settings_not_configured" />
 
+                <ImageButton
+                    android:id="@+id/btn_monitor"
+                    android:layout_width="44dp"
+                    android:layout_height="44dp"
+                    android:src="@drawable/ic_headset"
+                    android:background="?android:attr/selectableItemBackgroundBorderless"
+                    android:contentDescription="@string/btn_monitor"
+                    android:scaleType="centerInside" />
+
                 <TextView
                     android:id="@+id/lbl_tg"
                     android:layout_width="wrap_content"