[Development] Qt6 source changes

[Kevin Kofler]

Thiago Macieira wrote:
> Whether we'll have SSO is not decided.

That's good news, because I'm not convinced that that is a good idea, 
either. It should be done only if it really is a win.

But that also removes one major argument for changing QList to yet another 
QVector: the argument was that QStringList or QList<String> would be 
inefficient with SSO QString, but that is moot if QString stays as it is.

        Kevin Kofler