Move Value::fromBool, ... to a new Primitive class

Move Value::fromBool, ... to a new Primitive class

This will simplify finding the remaining direct usages of
QV4::Value that need fixing.

Change-Id: I223099727436d5748027c84c53d9dfc4028e38ed
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>