Add wxFileDialog::GetCurrentlySelectedFilename().

Also send wxEVT_UPDATE_UI events for the extra controls in wxFileDialog.

The combination of these changes allows extra controls to update their state
depending on the current selection in the dialog. Show a simple example of
doing it in the dialogs sample.

Closes #15235.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74071 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/filedlg.h

@@ -183,6 +183,26 @@ public:
     */
     virtual ~wxFileDialog();
 
+    /**
+        Returns the path of the file currently selected in dialog.
+
+        Notice that this file is not necessarily going to be accepted by the
+        user, so calling this function mostly makes sense from an update UI
+        event handler of a custom file dialog extra control to update its state
+        depending on the currently selected file.
+
+        Currently this function is fully implemented under GTK and MSW and
+        always returns an empty string elsewhere.
+
+        @since 2.9.5
+
+        @return The path of the currently selected file or an empty string if
+            nothing is selected.
+
+        @see SetExtraControlCreator()
+    */
+    virtual wxString GetCurrentlySelectedFilename() const;
+
     /**
         Returns the default directory.
     */