RFR: 8308644: [Linux] Simplify and fix small bugs on glass key related events [v4]

[Thiago Milczarek Sayao]

On Wed, 24 May 2023 00:54:27 GMT, Thiago Milczarek Sayao <tsayao at openjdk.org> wrote:

>> This PR adds missing key mappings, fixes "Alt Gr" for some systems and cleans `process_key`a little bit.
>
> Thiago Milczarek Sayao has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Forgot test code
>  - Back to hash

Before (when pressing dead keys):
![image](https://github.com/openjdk/jfx/assets/30704286/1172757a-7a3c-4a95-90da-8989f70a2ca7)

After:
![image](https://github.com/openjdk/jfx/assets/30704286/b1b02f96-9239-433d-8954-74433261c01e)

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PR Comment: https://git.openjdk.org/jfx/pull/1143#issuecomment-1561161467