Change Message::Flags/Type cast to Int, not UInt

Use toInt() for casting `Message::Flags` and `Message::Type`, instead
of toUint().

MSVC has trouble with uint-based enums, so Quassel switched to int
enums.  However, not all of the enum casts were converted, resulting
in needless unsigned-to-signed casting.

Thanks to @justJanne for reporting this.