v1.20.1

This commit fixes some bugs introduced since v1.19.1. These regressions
only appear when compiling for Windows:

• Fix 70b615f: "Bug fix: path method not appropriate for URLs"
  (bug introduced in commit 97d2be7)
• Erroneous path handling when compiling for Windows:
  • The .is_absolute() path method acts differently under Windows.
    Fix in: c648342
  • Under Windows the std::path::Components Prefix() and RoodDir
    need special care. Fixes in: 632e31a and 3ea8de2
  • Rust's Windows Paths optionally have \ as path separator:
    Fix in: 0b61979

As mentioned above, these fixes only concern the proper function of
the viewer when compiled for Windows. None of the bugs have security
implications. Nevertheless, I decided to yank the whole v1.19 series
because for a good user experience the viewer is expected to work.