Make QExplicitlySharedDataPointer<T> copy-ctor from QESDP<X> more safe

With "QExplicitlySharedDataPointer::QExplicitlySharedDataPointer( const
QExplicitlySharedDataPointer<X> & other)" implicitly doing an
static_cast<T *>(...) on other.data(), this could lead to dangerous use
of this copy constructor.

Example code:
  QExplicitlySharedDataPointer<Base> base(new Base);
  QExplicitlySharedDataPointer<Derived> derived(base); // that works!

This patchs disables the use of the static_cast, and adds a new define called
QT_ENABLE_QEXPLICITLYSHAREDDATAPOINTER_STATICCAST to re-enable
that code path.

Note, that the other way-around (assigning 'derived' to 'base') still
works as intended.

Other side note: QtXmlPatterns is relying heavily on the hidden
static_cast "feature". The other default Qt modules compile fine with
the static_cast removed.

[ChangeLog][Important Behavior Changes] QExplicitelySharedDataPointer's
copy constructor which performs a static_cast from "X *" to "T *"
(when constructing a QExplicitlySharedDataPointer<T> from
a QExplicitlySharedDataPointer<X>) doesn't perform a static_cast
from "X *" to "T *" any more. Instead, an implicit cast is now
performed. This change will break compilation of code that
relied on the downcast (i.e. cast towards a more derived type) of the
templated type when copy costructing a QExplicitelySharedDataPointer
object. Please refer to the class documentation for more information
about this issue and a workaround to keep old code compiling.

Change-Id: Id32aba6cda4e6d44728d7bc3a5c0c7a20f19adc6
Reviewed-by: Kevin Funk <kevin.funk@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>