Do not hardcode font for street styles to Arial, but use generic map font

Summary:
* make static methods member methods for access to members
  (e.g. default font member)
* remove default arg value QFont(QLatin1String("Arial")
* reuse same QFont object more (implicitely shared data)

Seems vector tiles are cached somehow, so a new font is
not picked up instantly

Reviewers: sanjibanb, nienhueser, #marble

Reviewed By: nienhueser, #marble

Differential Revision: https://phabricator.kde.org/D2598