UI: Display a spinner during initialization

[let-def]

On first start, TeXpresso downloads necessary TeX/LaTeX files from Tectonic repository.
This can take some time, during which TeXpresso appears to be frozen.
We should display some information or a spinner to keep users waiting.

One difficulty is that there are two sources of downloads and none of them have an easy to display information:

• from tectonic process downloading packages and metrics
• from renderer downloading graphical resources (mostly fonts)
  On tectonic side, we have no direct connection to the UI. On renderer side, we are in a synchronous function waiting to load a font, from which it would be difficult to update the window.

As a first approximation, I am thinking to display a simple spinner with a fixed message when there is nothing else to display (document is being processed and no page has been produced yet).