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/file.h

@@ -295,8 +295,11 @@ public:
         Get back a file descriptor from wxFile object - the caller is responsible for
         closing the file if this descriptor is opened.
         IsOpened() will return @false after call to Detach().
+
+        @return The file descriptor (this is new since wxWidgets 3.0.0, in the
+        previous versions this method didn't return anything).
     */
-    void Detach();
+    int Detach();
 
     /**
         Returns @true if the end of the file has been reached.