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

[Glavo]

On Sun, 2 Apr 2023 20:38:25 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`.

I am a native speaker of Chinese. I tested it using the ibus pinyin input method on Ubuntu. I found some problems:

* Input method candidate window sometimes appear in the wrong location:

![image](https://user-images.githubusercontent.com/20694662/229506031-ee5d6061-8405-4e81-8602-efb51d6d14ee.png)

* When the focus moves away from a text field, the console will continuously output an assertion error log:


(java:3296): Gtk-CRITICAL **: 20:18:35.210: gtk_im_context_set_cursor_location: assertion 'GTK_IS_IM_CONTEXT (context)' failed

It appears to be in the correct position, but another issue still exists.

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PR Comment: https://git.openjdk.org/jfx/pull/1080#issuecomment-1494222748
PR Comment: https://git.openjdk.org/jfx/pull/1080#issuecomment-1497697683