Move MessageHandler::sendMessage() into MessageComposition

MessageHandler::sendMessage() could only handle very specific messages. You could only set the recipient, body and the spoiler of the message. It was necessary to have such a simple function that could be called from QML previously.

However, since there is the MessageComposition class that can also be accessed from QML via properties to construct a complex message, we don't need the simple function anymore. MessageComposition can construct the Message object and send it via sendPendingMessage().

This also avoids some little redundancy. In the file sharing case a more complex message already needed to be created. Now everything shares the same code in MessageComposition.