Rename "hangul_mode" property to "InputMode".

And Add symbol to InputMode property.
This will show InputMode status on gnome-shell.
Add "initial_input_mode" config item.

If initial_input_mode is "hangul", then hangul engine will start
in hangul mode, otherwise in latin mode.
I've changed hangul_mode to more general name, input_mode.

I will add UI for this config item later.