Add wxDictionaryStringSortAscending comparison function.

Add "dictionary sort" callbacks and document them and the already existing
wxStringSortAscending() and wxStringSortDescending().

See #16330.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76753 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775