[backend] Drop unused function
[tests] Use proper minus here

Though MathML recommends that hyphen-minus is treated as minus in math
mode, I think this is nonsense and authoring tools should use the proper
Unicode code point of the minus.
[tests] Encode the prime properly

The rendering is not so good, though.
[tests] Use mathvariant, not fontstyle/fontweight
[tests] Use the asterisk operator
[tests] Remove useless test
[backend] Plug some memory leaks
[engine] Plug a memory leak
[backend] Let HarfBuzz shape the whole string

This prepares for applying ‘ssty’ or ‘dtls’ features in the future. Does
not handle glyph positioning yet, though.
[backend] Use m_ prefix for member variables
[backend] Just pass the FormattingContext at once

Instead of an increasing list of arguments.
[backend] Apply ‘ssty’ feature to script level > 0
[build] Ignore .DS_Store file OSX
[qt] Use HarfBuzz in Qt backend
[qt] Update Qt port README
[qt] Don’t spam the terminal with debug messages
[qt] Drop the need for private Qt headers

Instead of trying to get the underlaying hb_font_t that Qt does not
expose, we create a new one from the font data.
[qt] Update the instructions