This changes `ShareForumActivity` to use two fragments to facilitate
forum sharing with the new Forum Sharing Client backend.

The `ContactSelectorFragment` allows the user to select a
number of contacts. If there is an ongoing sharing session or the forum
is already shared with the contact, it is disabled in the list. If there
is at least one contact selected, a button appears in the toolbar that
brings the user to the `ShareForumMessageFragment` where the user can
write an optional message to be send along with the invitation.

After sending an invitation, the user is brought back to the forum that
she shared and there is a snackbar showing up briefly to indicate the
successful invitation.

The invitation is shown along with the message within the private
conversation of each contact. The person who shares the forum also sees
the invitation and the message as outgoing messages that also display
the current status of the messages.

A notification is shown like for other private messages as well.

Please note that this commit does not include a way for users to respond
to invitations.

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