When a child process sends a documentsizechanged: message to the parent, to be
forwarded to the client, parse it and update a cached status of the doucment,
if available, and send an updated status: message to the client instead.
Note that clients should not rely on getting only status: messages and never
documentsizechanged: messages, though; the cache might be cleaned at any time
even while the server is running. If there is no cached status of the document
to update and re-use, we have to forward the documentsizechanged: message as
such to the client.
The JS code always passes in 0 for now. The server parses the parameter and
calls LibreOfficeKitDocument::setPart() before calling paintTile().
Probably also the status, key, mouse and selection messages will need a part
number. The intent is after all that the protocol is as stateless as
possible. (So maybe we should also pass the document URL in each message?)
