QFixed{,Size}: reformulate some functions in a constexpr-friendly way

QFixed{,Size}: reformulate some functions in a constexpr-friendly way

The main change is the addition of a new constructor
that passes its argument into 'val' verbatim. In order
to disambiguate it from the existing QFixed(int)
constructor, it takes a second 'int' argument. This
is too ugly for public API, so it's private, and only
used by static QFixed fromFixed(int), which is the
existing named constructor with the same semantics.

The rest of the changes simply reformulate their
operations in terms of fromFixed().

This makes them ready to be constexpr'ed.

Change-Id: I2a3813d62bd4124064755de6b00526a60fc82c1d
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>