Expose Qt.application.font

This is useful in order to specify font sizes that are some multiple of the default size (which is almost always guaranteed to be a sensible, legible size). For example:

Text {
    text: "Large text"
    font.pixelSize: Qt.application.font.pixelSize * 2
}

Other options are:

1. "<h3>Large text</h3>" - adds extra space below the text since it's a HTML element, so not really useful if you want a decent layout.
2. Hard-coding a pixel size. Works OK when used in combination with Qt::AA_EnableHighDpiScaling, and so long as you guess the correct size that works for each platform you're targeting. Doesn't work without setting Qt::AA_EnableHighDpiScaling.
3. Using FontMetrics/TextMetrics. Works fine, but is a bit verbose.
4. Use an empty Text element. Creates an unnecessary item. Was superseded by FontMetrics/TextMetrics.
5. defaultPixelSize/implicitPixelSize and defaultPointSize/implicitPointSize. There are already font-related properties outside of the font grouped property, so this wouldn't be out of place, but the API is already quite large.