QEasingCurve: simplify and fix copy constructor

QEasingCurve: simplify and fix copy constructor

The copy constructor used the default Private constructor,
followed by the application of the compiler-generated
copy assignment operator, and finally replaced the config
member with a copy of itself.

This is needlessly inefficient.

Worse: it's incorrect: if config->copy() throws, then
*d_ptr is leaked.

Solution: implement the copy constructor for Private,
and use it in the copy constructor of the public class.

Effect: everything that can throw now prevents the new
Private class from being created, and the compiler ends
up cleaning up after us.

Change-Id: I09ed18bb39ee7cd81aaa8ba01676fc202502a8e3
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>