Add wxFileDialog::GetCurrentlySelectedFilename().

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