Support app-level default fonts

[brandonscript]

App-level fonts can be defined, but require a font size. Need to support an app-level default font that doesn't need defining on each text control. E.g.:

{
    "$app.font.family": "Aileron",
    "$app.font.weight": "Light",
    "$app.font.size": 16,
}

And then on a text control, changing the size is as easy as:

{
    "_type": "Text",
    "font.size": 18,
}