Support rich text in rtk.Font

[DaveInDev]

following
I've thought about adding some sort of rich text mechanism to the low-level rtk.Font class that everything in rtk uses to render text, and that would support something like this, but it's quite a lot of work and there's more pressing problems to solve so I haven't done it yet. But again I invite you to open an issue on GitHub for that.

Could be nice to have buttons in italic, or bold, or just a letter underlined to show it's shortcut...
Bu I agree that it's a big piece of work.