Add getGroupName method

src/main/java/org/asamk/Signal.java

@@ -23,6 +23,8 @@ public interface Signal extends DBusInterface {
 
     void setContactName(String number, String name);
 
+    String getGroupName(byte[] groupId);
+
     class MessageReceived extends DBusSignal {
         private long timestamp;
         private String sender;

src/main/java/org/asamk/signal/Manager.java

@@ -785,6 +785,11 @@ class Manager implements Signal {
         }
     }
 
+    @Override
+    public String getGroupName(byte[] groupId) {
+        return getGroup(groupId).name;
+    }
+
     @Override
     public void setContactName(String number, String name) {
         ContactInfo contact = contactStore.getContact(number);