Improvements for RTL languages

This PR addresses a few issues for RTL (right to left) languages I encountered when reviewing #3680. This PR solves:

- An issue that a horizontal scrollbar was shown for an RTL language. This was because `margin-left` had to be `margin-right` for RTL languages at https://github.com/timvandermeij/pdf.js/commit/67e4c03786910fadd8cf2bf4d170105ce742efaa#L3R452.

- Some icons in the viewer were not mirrored properly, which needs to be the case for RTL languages. This was the case for the secondary toolbar toggle icon, the sidebar toggle icon and the view outline icon.

- At last, some very minor coding style issues were fixed in the parts of the CSS that I touched.