Add support for arbitrary NSViews in QMacToolBar

I needed support for adding NSViews to a QMacToolBar, so I wrote it myself. I'm not familiar with Qt contribution model. For the moment I have a fork on GitHub. Here's a link to the code:

https://github.com/flaviotordini/qtmacextras-1/commit/9e15f2ec1c5e786e2a629caf2563ed7610a26c49

If this is interesting for you, please guide me in getting this in QtMacExtras.