Implement support for units (px, pt, percent etc) in QML

Ideally we want to write something like:

Text {
    fontSize: 12pt
}

How do we get there? Already today we can do the following:

1. Write constructible value types called "pt" and "px" that can be constructed from a number.
2. Write a value type called "screenRelatedSize" (please bikeshed the name) and make it constructible from pt or px, basically a tagged union over those, which can be resolved into some physical size if given a dpi value.
3. Add a fontSize property of type screenRelatedSize to Text and have it handle whatever that produces.

Now you can write:

Text {
    fontSize: 12 as pt
}

That's already not all that bad.

To improve on this even further, we could add a syntactical hack to the QML parser so that a number directly followed by a lowercase identifier (without space in between) is interpreted as an "as" expression. This should be possible because such a thing is currently invalid code AFAICS. With that, we can have the original form.

An argument can be had that we shouldn't actually squat such a prominent syntactical construct in case JavaScript wants to add it it itself, with different semantics. In that case we may want to either opt into this using a pragma or offer some less obvious syntax like "12_pt".