Add wxSecretStore

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).

include/wx/android/setup.h

@@ -433,6 +433,13 @@
 // Recommended setting: 1 (but may be safely disabled if you don't use it)
 #define wxUSE_FSVOLUME      1
 
+// Use wxSecretStore class for storing passwords using OS-specific facilities.
+//
+// Default is 1
+//
+// Recommended setting: 1 (but may be safely disabled if you don't use it)
+#define wxUSE_SECRETSTORE   1
+
 // Use wxStandardPaths class which allows to retrieve some standard locations
 // in the file system
 //