selkey setting is not handled natively

[kanru]

After porting windows-chewing-tsf from chewing_handle_Default to chewing_handle_KeyboardEvent, selkey setting stopped working.

It was because the selkey setting was enabled by a hack in chewing_handle_Default.

See #763 for short-term fix.

We should support the selkey setting directly from chewing-rs, possibly using the new keymap framework.