Handle white space correctly in Dotify-based renderer

[bertfrees]

"Correctly" means conforming to the braille CSS standard.

See also:

• White Space Processing section in OBFL specification

Related issues:

• https://github.com/snaekobbi/braille-css-spec/issues/30
• https://github.com/joeha480/obfl/issues/25

[bertfrees]

The CSS white space processing and line breaking rules (see White Space Processing Rules, Line Breaking and Breaking within Words) will be implemented as follows:

• White space is preserved in pxi:css-to-obfl:
  • Preserved spaces are converted to sequences of non-breaking spaces followed by a zero-width space.
  • Preserved line breaks are converted to <obfl:br/>
• Dotify handles collapsible white space in an OBFL preprocessing step (OBFLWsNormalizer)
• Line breaking happens in ByPassTranslator. It will correctly handle non-breaking spaces, zero-width spaces and soft hyphens.

[bertfrees]

Rebased onto master and merged.