First part of font refactoring. These changes simplify a few things

[glennw]

but don't do much on their own, they just make it easier to
implement the work to come (web fonts, performance improvments
in terms of font loading and memory usage).

• Font identifier on Linux/Android is now the font file path.
  This is a temporary measure, but simplifies things a lot for now.
• Remove FontListHandleMethods trait in favour of free functions.
• FontList::refresh() has no knowledge of FontFamily etc. Instead it takes
  a closure that the caller provides.
• FontList::load_variations_for_family no longer creates the font
  handle. Instead it takes a closure and provides the name of the font
  identifier for the variations it finds.
• Remove path_from_identifier() - it's no longer required.
• create_font_from_identifier() takes an Option<Style>, allowing it to be
  used to create fonts for family matching purposes where the font size is
  not important.

Tested on Linux + Mac. Builds on Android but not able to confirm it's working correctly.

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[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[glennw]

r? @metajack @larsbergstrom

[larsbergstrom]

r+