Close a modal dialog even when it doesn't have any buttons.

Close a modal dialog even when it doesn't have any buttons.

The close button in the dialog title bar should work even if there are no
buttons in the dialog itself (unlike the Escape key which works as an
accelerator for a button), so close the dialog explicitly if the emulated
button click wasn't processed in wxDialogBase::OnCloseWindow().

Closes #12513.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65653 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775