sizeof(QFlags<Enum>) should equal sizeof(Enum)

slightly related to QTBUG-111926

Often enough, QFlags may not be particularly large. For example when 8 options is enough, the underlying enum type could just be a `std::uint8_t`. Wrapping the enum type in QFlags unfortunately results in an type four times as big! This is obviously wasteful! Instead, sizeof(QFlags<Enum>) == sizeof(Enum) should always hold.