Must clarify QML dimension units when using AA_EnableHighDpiScaling

As opposite to other layout engines (CSS, Android layout etc.), QML doesn't have a concept of dimension units like px, pt and dp.

If we define a rectangle like this

Rectangle {
  width: 100
  height: 100
}

we will obtain a square of size 100*scale px if we set AA_EnableHighDpiScaling.

Thus the dimension unit is dp instead of true pixels in this case. Which is judicious is most cases. But it is implicit.

I don't know if we should add units to QML, but at least we should describe how numbers are interpreted by the framework. And more generally how to achieve responsive design with Qt.

Same apply for font, what is the definition of pointSize and pointPixel?