find a way to allow large QWSCommands

Currently we close the socket and get undefined behavior if a QWSCommand is too large. This happens easily if you select and copy a large chunk of text in a client application.

• Split into several commands?

We do have a cutoff on the client side, so Qt never sends commands that are too large. However,
truncation may cause malformed commands... but that should now be handled by the "hardening"
code in change 256269.