Modernise QT_DEPRECATRED(_X) mechanism - use [[deprecated]] internally

We have a platform-dependent macro expansion for QT_DEPRECATED(_X) when there's a C++14 attribute standardised [1].

This is important for certain corner cases that are supported by [1] but not supported by platform-specific extentions: I stumbled upon an issue trying to deprecate an enum value, e.g.

enum {
  NewValue,
  OldValue QT_DEPRECATED_X("foobar"),  // causes compile errors now
  OtherValue,
};

[1]: https://en.cppreference.com/w/cpp/language/attributes/deprecated