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

include/wx/msw/filedlg.h

@@ -44,6 +44,9 @@ public:
     // called from the hook procedure on CDN_INITDONE reception
     virtual void MSWOnInitDone(WXHWND hDlg);
 
+    // called from the hook procedure on CDN_SELCHANGE.
+    void MSWOnSelChange(WXHWND hDlg);
+
 protected:
 
 #if !(defined(__SMARTPHONE__) && defined(__WXWINCE__))