- Simplify how we look-up the DOM-element, which should also be a tiny bit more efficent.
- Use private class-fields, rather than property-names prefixed with underscores.
- Inline the `#updateBar` helper-method directly in the `percent`-setter, since having a separate method doesn't seem necessary in this case.
- Set the `indeterminate`-class on the ProgressBar DOM-element, to simplify the code.
Finally, also (slightly) re-factors the `PDFViewerApplication.progress`-method to make it a bit smaller.
The original `ProgressBar`-functionality is very old, and could thus do with some general clean-up.
In particular, while it currently accepts various options those have never really been used in either the default viewer or in any examples. The sort of "styling" that these options provided are *much better*, not to mention simpler, done directly with CSS rules.
As part of these changes, the "progress" is now updated using CSS variables rather than by directly modifying the `style` of DOM elements. This should hopefully simplify future changes to this code, see e.g. PR 14898.
Finally, this also fixes a couple of other small things in the "mobile viewer" example.
