Allow user program to redirect/hide QML warnings

The different items & bindings use QDebug (specifically: QDeclarativeInfo, which wraps QDebug) to place runtime warnings. However, there is currently no way to redirect or silence these runtime warnings. I suggest providing an option to do this.

Background: When working on QmlDesigner we experienced that there are lots of QtDeclarative runtime warnings that do not do any harm (program still works as expected), and are hard to fix. Furthermore, we want to separate between warnings that are in QtCreator itself, or warnings that are within the user .qml file we edit. I expect that other programs will want to redirect the output, also.

The redirection should ideally be configurable per QDeclarativeEngine (something like QDeclarativeEngine::setWarningSink(QIODevice))