RFR: 8305418: [Linux] Replace obsolete XIM as Input Method Editor [v6]

[Thiago Milczarek Sayao]

On Mon, 18 Sep 2023 00:34:17 GMT, Thiago Milczarek Sayao <tsayao at openjdk.org> wrote:

>> This replaces obsolete XIM and uses gtk api for IME.
>> Gtk uses [ibus](https://github.com/ibus/ibus)
>> 
>> Gtk3+ uses relative positioning (as Wayland does), so I've added a Relative positioning on `InputMethodRequest`.
>> 
>> [Screencast from 03-09-2023 19:10:36.webm](https://github.com/openjdk/jfx/assets/30704286/2a36e9d0-59be-4092-905f-e31fabc6e2e4)
>
> Thiago Milczarek Sayao has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix highlight

I think it's usable now. I'm not really sure because I'm on unknown land here :).

To my understanding the gtk ime system will use only one preedit attribute which I detect on pango attribute list where:

- `PANGO_ATTR_BACKGROUND`: it's a IME preedit suggestion and must be highlighted 
- `PANGO_ATTR_UNDERLINE`: it's a dead key

otherwise it's a normal input (probably will never hit this case on preedit as it will go to commit directly).

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PR Comment: https://git.openjdk.org/jfx/pull/1080#issuecomment-1722621185