Fix: Use QFontDatabase::systemFont() for default fonts

[filiperinaldi]

When building for QT5.2 and later, use QFontDatabase::systemFont() to retrieve the default fonts. This fixes two font issues on the MacOS:

• The fonts selected via QApplication::font() are not optimum and look very pixelated.
• QFont::setStyleHint() doesn't retrieve a valid font though QFont::setFixedPitch() enforces a fixed pitch. This results in an unreadable diff view.