QEasingCurve: implement move constructor

QEasingCurve: implement move constructor

The move constructor sets other.d_ptr to zero. This is safe, because
after being moved from, the object is left in a state in which it
can be safely destroyed (delete nullptr is a no-op).

It cannot meaningfully be used anymore (most members will crash with
a nullptr dereference), but in most cases, the moved-from object
cannot be accessed anyway (not a named object), and if a named object
is moved from, it must have been through explicit std::move(), as in
the test case.

The STL makes better guarantees (moved-from containers are .empty()),
but I don't think it's worth introducing a null state into
QEasingCurve just for supporting a use-case that should be
considered a bug anyway.

Change-Id: I4115b7386cdea6960507da6843a0d0196d8e4139
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>