Fix checking for the deepest shown window in focus handling code

wxSetFocusToChild() could recurse to the parent TLW, which was clearly
nonsensical as it means that restoring last focus in a dialog could end
up setting it to the parent frame.

Fix this by breaking out of the loop as soon as we reach a TLW.

See #18653.

src/common/containr.cpp

@@ -711,6 +711,9 @@ bool wxSetFocusToChild(wxWindow *win, wxWindow **childLastFocused)
                 else
                     deepestVisibleWindow = NULL;
 
+                if ( (*childLastFocused)->IsTopLevel() )
+                    break;
+
                 *childLastFocused = (*childLastFocused)->GetParent();
             }