[mythtv] Ticket #5311: Write QString replacement

[Jerome Brock]

I have made an attempt at writing a thread-safe QString replacement:
MString, and attached a patch to this ticket.

If there is positive reception to this approach, I am willing to start
work on a thread-safe replacement for QStringList: MStringList.

Just let me know.

Some notes on the design of MString:

It can't inherit from QString as none of QString's methods are declared
virtual, thus killing an attempt at polymorphism.

Also, it is designed for concurrent access by multiple readers or one
writer.

Jerome