This adds a sign() and a verify() method to the CryptoComponent
that take a mandatory label argument to ensure that signatures can't be
repurposed.

Use namespaced strings for Transport IDs

Closes #558

See merge request !420

Do not track incoming positive introduction responses

Positive introduction responses are not shown in the UI (for introducees) and are therefore not marked as read. If they would be tracked, the unread message count would be higher than it actually is and would never decrease.

This is a minimal fix that could be better, but I didn't bother to refactor anything, because we need to rewrite the introduction client eventually anyway once more.

Closes #759

See merge request !416

because they are not shown in the UI and are therefore not marked as
read. This fixes the unread message count.

Remove unnecessary unmodifiable collection wrappers

Part of #379.

See merge request !414

Unit tests for some validators

See merge request !413

Use new group visibility state to avoid lost messages

Depends on !410. Closes #756.

See merge request !411

Add third group visibility state

This branch adds a third group visibility state: each group is either invisible, visible, or shared with respect to each contact.

Invisible means that the contact doesn't see any sign that we subscribe to the group. Visible means that the contact can send us messages in the group, but we won't send the contact messages in the group. Shared means that the contact can send us messages in the group and we'll send the contact any shared messages in the group.

This is a preparatory step for #756. I'll put up another MR with the changes that close that ticket.

See merge request !410

Use MessageTracker for private group invitation messages

Depends on !405.

See merge request !412

Factor MessageTracker out of BdfIncomingMessageHook

The branch moves the MessageTracker implementation from BdfIncomingMessageHook to a separate class. This will allow the private group invitation client to track messages from classes other than the one that implements the delivery hook.

I've also fixed a couple of bugs, removed some redundant code from the validation manager, and added null-safety annotations (which is how I noticed the bugs).

See merge request !405

Add visibility and OPTIONS button to private group join notices

![device-2016-11-11-180658](/uploads/e00d175a1e1f34307c5f0d80fa0d1cdf/device-2016-11-11-180658.png)
![device-2016-11-11-181325](/uploads/0f6010094d529a4f151db8ebce974885/device-2016-11-11-181325.png)

Closes #732

See merge request !408

Made private group timestamp greater than that of latest message

I did not change `getPreviousMsgId()` to `getPreviousMsgHeader()` because there doesn't seem to be a need for it anymore.

Closes #755

See merge request !409

Add onboarding dialog for revealing contacts screen

This MR is based on !406.

![device-2016-11-11-162417](/uploads/8df831905eebf41ecdbe368890cdd429/device-2016-11-11-162417.png)

See merge request !407

Add UI for revealing contacts within a private group

This addresses one part of #732. Join notices and onboarding will follow in separate MRs.

Although this MR is technically not based on !402, it does require it to be merged first to compile.

![device-2016-11-11-160454](/uploads/8cb6d8bdd0f3761657875024739e730a/device-2016-11-11-160454.png)

See merge request !406

Mark relationship visible when syncing group with peer

This branch updates the private group invitation protocol to use @grote's new method for marking a contact relationship visible to the group.

I've changed the method slightly because the protocol state machine allows us to leave and re-enter the BOTH_JOINED state (see diagram on #659), so the relationship may already be visible when the method is called. In that case the visibility isn't updated, so we stick with whichever of revealed-by-us and revealed-by-contact happened first.

See merge request !402

Prepare UI for revealing contacts

This changes the visibility of some methods (that need to be accessed from another package), removes unnecessary abstractions and fixes erroneous static import of GroupId constant.

See merge request !404

This changes the visibility of some methods, removes unnecessary
abstractions and fixes static import of GroupId constant.

Add new Exception handler

While working on #732 I again needed to use a `UiResultExceptionHandler`  when I actually don't need to return a result. We have some other places in the code like this. So I introduced a `UiExceptionHandler` without the result part and used it where appropriate. While I was touching some classes, I also added the new annotations.

This MR includes another small commit that notifies only on local group messages. Joining a group is a message as well and without this change, you are notified about a new message when you yourself joined your newly created group.

See merge request !403