Splash screen transparency on Mac 

This corrects a bug in latest nightly releases that made the splash screen not transparent on MacOS. If you download any of the latest medinria nightly builds for Mac, you'll see the splash screen is not transparent anymore since we changed the QSS. 

The easy way to solve this was to make the splash screen derive from QWidget instead of QFrame. And that's what I did here. I tested on a macleod build and it works