Add qVar() global function for creating QVariants

QVariant can hold a value of (almost) any type, given the requirements are met, but there is no easy, universal way of creating a QVariant of any value.

The value is either basic type, and Qt has a easy, non-explicit c-tor for it.

or

The value is of custom type and the user must use the clunky and verbose QVariant::fromValue().

My suggestion is to add a simple function "qVar()" to be able to easily create a qvariant from any value with a simple and concise syntax.

Usage:

auto var = isCustom ? qVar(getCustom()) : qVar(getStandard());

Usage: 

QVariant getValue(Data data, Type type)
{
  switch(type)
  {
  case type_int:
    return data.someIntValue();
  case type_string:
    return data.someStringValue();
  case type_custom:
    return qVar(data.someCustomValue());
  }
}

The implementation is trivial:

template<class T>
QVariant qVar(T&& val)
{
  return QVariant::fromValue(std::forward<T>(val));
}

One interesting side effect is that the user can override the function for his own type:

QVariant qVar(const Custom& c)
{
 // magic
}