Address related improvements

- QR code is now shown in contact details and 'manage identity' view
- Contacts and identities can now be deleted

app/src/main/java/ch/dissem/apps/abit/service/Singleton.java

@@ -124,4 +124,10 @@ public class Singleton {
         }
         return identity;
     }
+
+    public static void setIdentity(BitmessageAddress identity) {
+        if (identity.getPrivateKey() == null)
+            throw new IllegalArgumentException("Identity expected, but no private key available");
+        Singleton.identity = identity;
+    }
 }