Return the old file descriptor/pointer from wx(F)File::Detach().

Make Detach() more convenient to use.

Closes #15494.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74798 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/ffile.h

@@ -76,8 +76,11 @@ public:
         closing the file if this descriptor is opened.
 
         IsOpened() will return @false after call to Detach().
+
+        @return The FILE pointer (this is new since wxWidgets 3.0.0, in the
+        previous versions this method didn't return anything).
     */
-    void Detach();
+    FILE* Detach();
 
     /**
         Returns @true if an attempt has been made to read @e past