[nemo-qml-plugin-messages] Build for Qt5

MessagesContextProvider is currently disabled, as contextkit doesn't
exist for Qt5. That will need to be refactored along with
commhistory-daemon to use statefs, a direct dbus service, or some other
method of communicating the current conversation state.
[nemo-qml-plugin-messages] Fix a possible assertation when creating channels

The client handler can receive the channel created by a channel request
before the channelRequestSucceeded signal arrives, in which case it will
call setChannel directly (done primarily for un-requested channels).

Fixed those code paths to have the same behavior and not trigger an
assertation on the state of the request.