Add single-shot connection type

Sometimes, it is useful to have a slot just be triggered once when a signal fires, and then stop listening. There are of course several ways to do that, but they all require a bit of code to keep track of the connection manually or even a second connection to trigger the disconnect of the first.

It would be convenient to have a special connection type for this: Qt::singleShotConnection. The idea would be that specifying this as the connection type, the connection would be automatically broken after the first time it was triggered.