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/filedlg.h

@@ -122,6 +122,9 @@ public:
     virtual wxString GetWildcard() const { return m_wildCard; }
     virtual int GetFilterIndex() const { return m_filterIndex; }
 
+    virtual wxString GetCurrentlySelectedFilename() const
+        { return m_currentlySelectedFilename; }
+
     // this function is called with wxFileDialog as parameter and should
     // create the window containing the extra controls we want to show in it
     typedef wxWindow *(*ExtraControlCreatorFunction)(wxWindow*);
@@ -155,6 +158,13 @@ protected:
     wxString      m_fileName;
     wxString      m_wildCard;
     int           m_filterIndex;
+
+    // Currently selected, but not yet necessarily accepted by the user, file.
+    // This should be updated whenever the selection in the control changes by
+    // the platform-specific code to provide a useful implementation of
+    // GetCurrentlySelectedFilename().
+    wxString      m_currentlySelectedFilename;
+
     wxWindow*     m_extraControl;
 
     // returns true if control is created (if it already exists returns false)