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

[Thiago Milczarek Sayao]

While working on IME I noticed this code could be simplified, so I removed the `g_hash_table` usage for a simple struct ordered for binary search (I'm sure it's ordered because I did a simple app to generate it ordered).

This also fixes:

- UNKNOWN key codes being sent on KEY_PRESS;
- Alt Gr is now working;
- Replaced X calls with GDK calls for key locked check for as a small step towards supporting Wayland in the future
- Added missing mappings (for F13 - F24 and dead keys) - Linux does not send dead keys as keypress but sends as key release, and they might be used on a robot test, so it's now covered.

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Commit messages:
 - Remove print statement
 - Use GDK instead of X calls (to ease in a future wayland implementation)
 - Avoid sending unknown key event
 - GDK_META_MASK is not ALT
 - Map Alt Gr key which is named GDK_KEY_ISO_Level3_Shift
 - Remove duplicated entry
 - Add missing mappings
 - Remove unused preprocessor
 - Add note
 - Replace g_hash_map for a binary search

Changes: https://git.openjdk.org/jfx/pull/1143/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1143&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308644
  Stats: 320 lines in 3 files changed: 31 ins; 55 del; 234 mod
  Patch: https://git.openjdk.org/jfx/pull/1143.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1143/head:pull/1143

PR: https://git.openjdk.org/jfx/pull/1143