[nemo-qml-plugin-messages] Update MessageContextProvider for property changes

Added the ObservedInbox property, and changed the API and value of
ObservedConversation to allow multiple groups to be observed
simultaniously.

Requires commhistory-daemon 0.4.7.
[nemo-qml-plugin-messages] Add rpm packaging
[nemo-qml-plugin-messages] Allow specifying an existing commhistory event ID for outgoing messages

commhistory-daemon will use the x-commhistory-event-id header in
messages to avoid creating duplicate events for outgoing messages.