[Development] QTBUG-23634: Fix QMap implementation

[lars.knoll at nokia.com]

This is my own bug. Same rules as for containers branch applies here. Can
go into api_changes as long as it's fully SC. Deadline is end of next week.

Just to mention the current status: I have the new QMap implementation
working (and passing the tests) with the exception of insertMulti(). I
have good hopes that I'll get it done until end of next week.

Cheers,
Lars