Advanced font management and browsing

Most of the time font selection takes way too much time. Mostly I don't want to use fonts that is installed by different apps and the system. Graphite should have a better font selection ux.

- There should be a dedicated panel to manage font selection.
- Ways to find fonts quickly that matches certain styles.
- Even better a separate modal (something like google fonts UI) to sort, group fonts by available features like languages and weights etc. And hide/enable/disable set of font in a click of a button.

I think these feature is to broad could be built inside a plugin. I hope at least graphite can pave the way for plugins like this.

As a starting point graphite should be able to read a json file for list of fonts graphite is currently allowed to list inside font selection dropdown menu.

Related: #1340