This also introduces constants for the GROUP_ID, FORUM_NAME and
MIN_TIMESTAMP as they were used for the two buttons.

Closes #313

(except refactoring of conversation item classes)

Show system notification for successful introductions

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

Whether or not a class needs to run as a service is an implementation decision.

"synchLock" will become confusing when we have lots of objects with "sync" in the name.

bug#49 Fixed by ensuring that intents for different contacts/groups can be distinguished from each other when resolving PendingIntent refs

- replaced use of Object instance mutex with a private final Lock object
- replaced Object signaling with specific condition signalling