Will currently fail at runtime; requires a public key and a server onion.

* force decline when two of our own identities are introduced to each
  other
* throw away introduction requests to the same identity
  (impossible to trigger from UI)

Closes #284

(except refactoring of conversation item classes)

When devices' clocks are out of sync, it is possible that a response is
shown before the request. This commit makes sure that the timestamp of
responses is always later than the last message in the conversation.

Some wording could be misunderstood to thing introductions were
successful even though they were not. That has been clarified.

A new database transaction was created when getting contacts and local
transport properties. This has been changed to re-use the existing
transaction.

Also addresses minor issues found in review.

The code made the assumption that a session state can be identified by
the unique session ID. However, when multiple identities from the same
device are involved, there are two sessions with the same ID running on
the device.

Hence, a second identifying criteria has to be used to uniquely identify
the correct session. Here, the ID of the group was chosen.
Unfortunately, the session state can not be cached easily anymore
leading to a small performance penalty when getting all messages for the
UI.

Show system notification for successful introductions

Fixes #270.

@color/briar_text_primary is used in a lot of places other than the settings screen - if we want to use grey text in the settings screen we'll need to find another way.

Also fixed some misspelled resource names and included the colours from the Briar palette in color.xml.

Modified the project structure, removed module extension and went instead for a non-complete core dependency graph

Semi-encapsulated the core/api dependency graphs and created a proper structure to load eager singletons