Fixes 'no-text' bug in HTTP server's raw text

Such a big commit to provide a fix which really only involves a few
lines in the graphics builder. It would seem that the very first time
the BlockCharMono font is rendered it needs a little delay. So there's a
refactor here to use a callback when requesting a screenshot with text.

All the other code in this commit is just the scaffolding to try to fix
this issue. It's all good stuff in itself. Basically stricter code
triggers for the different stages of page load: tab load, DOM ready,
page ready. I actually wonder if the TTY page loads feel snappier now?

closes #46