QFlag constructors introduce ambiguity under strict compilation

When using MSVC 2012 (cl 17.0) or 2013 (cl 18.0) with the option "/Za", i.e. disable extensions, many types that use QFlag/QFlags fail to compile.

An example is the QDir class.

This originates from the classes declaring enumerations with a mix of positive and negative integers (usually a -1 is declared, e.g. QDir::NoFilter).

• The enumeration values are passed to the QFlags constructor.
• A QFlag is implicitly constructed for conversion.
• The QFlag has multiple constructors, for int and uint, and, because the enumeration matches both, this causes an ambiguity.

This was not an issue with Qt4 because QFlag only had an int constructor.