No real changes, just add a new platform-specific NewImpl() method instead of
making wxSecretValue ctor itself platform-specific.
This makes adding other ctors for this class simpler.
Add a new class allowing to store passwords and other sensitive information
using the OS-provided facilities.
Add implementations for all the main platforms, documentation and a new sample
(which contains an ad hoc unit test as the real unit test for this class would
probably be a bad idea as it wouldn't run in non-interactive contexts and
could show OS level dialog boxes if it did).