Merge branch '1585-new-messaging-client' into 'master'

...ava/org/briarproject/bramble/api/client/ClientHelper.java

...ava/org/briarproject/bramble/client/ClientHelperImpl.java

...ject/bramble/properties/TransportPropertyManagerImpl.java

...roject/bramble/sync/validation/ValidationManagerImpl.java

...oject/bramble/versioning/ClientVersioningManagerImpl.java

.../bramble/properties/TransportPropertyManagerImplTest.java

...roject/bramble/test/BrambleCoreIntegrationTestModule.java

...t/bramble/versioning/ClientVersioningManagerImplTest.java

...ject/briar/android/conversation/ConversationActivity.java

...ect/briar/android/conversation/ConversationViewModel.java

...rproject/briar/android/view/TextAttachmentController.java

...g/briarproject/briar/android/view/TextSendController.java

briar-android/src/main/res/values/strings.xml

...va/org/briarproject/briar/api/client/ThreadedMessage.java

.../conversation/event/ConversationMessageReceivedEvent.java

.../briarproject/briar/api/messaging/MessagingConstants.java

...rg/briarproject/briar/api/messaging/MessagingManager.java

.../org/briarproject/briar/api/messaging/PrivateMessage.java

...iarproject/briar/api/messaging/PrivateMessageFactory.java

...riarproject/briar/api/messaging/PrivateMessageHeader.java