Fonts api with Themes integrated

Fixes:
https://github.com/adobe/brackets/issues/8425
https://github.com/adobe/brackets/issues/8381

References:
https://github.com/adobe/brackets/pull/8305

I generally just do something like this:

```javascript
prefs.definePreference("fontSize", "string", DEFAULT_FONT_SIZE + "px").on("change", function () {
// do stuff
});
```