This undocumented "private" class was used for various windows UxTheme
functions which are available since WinXP. As wxWidgets 3.1 is XP+ it
does not make sense anymore to load the theme functions dynamically.
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).
