Make wxSplit(wxJoin()) idempotent for string ending with escape

Previously, splitting a string obtained by joining together array
with (any but last) elements ending in the escape character (normally
the backslash), didn't recover the original array because the separator
character following it in the resulting string was considered to be
escaped by wxSplit().

Fix this by escaping the trailing escape character itself.

Add a test confirming that this works as expected now, document this
behaviour and also slightly simplify wxSPlit() logic.

See https://github.com/wxWidgets/wxWidgets/pull/2311

Closes #19131.

interface/wx/arrstr.h

@@ -523,7 +523,18 @@ wxArrayString wxSplit(const wxString& str, const wxChar sep,
     If the @a escape character is non-@NULL, then it's used as prefix for each
     occurrence of @a sep in the strings contained in @a arr before joining them
     which is necessary in order to be able to recover the original array
-    contents from the string later using wxSplit().
+    contents from the string later using wxSplit(). The @a escape characters
+    themselves are @e not escaped when they occur in the middle of the @a arr
+    elements, but @e are escaped when they occur at the end, i.e.
+    @code
+    wxArrayString arr;
+    arr.push_back("foo^");
+    arr.push_back("bar^baz");
+    wxPuts(wxJoin(arr, ':', '^')); // prints "foo^^:bar^baz"
+    @endcode
+
+    In any case, applying wxSplit() to the result of wxJoin() is guaranteed to
+    recover the original array.
 
     @see wxSplit()