Main reason to support this old interface is because this is the only
protocol chromium (and effectively all electron app) that supports.
The protocol itself very similar to text-input-v2 with some minor difference.
So not hard to support by just duplicate some existing code. There might be
some unclear protocol design issue if kwin need to support multiple SeatInterface,
but for now it should be ok to assume there is only one seat.
Tested using fcitx5 against weston-editor and chromium with flag
--enable-features=UseOzonePlatform --ozone-platform=wayland
--enable-wayland-ime
