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

[Thiago Milczarek Sayao]

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`.

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Commit messages:
 - Don't highlight for dead keys
 - Don't move the caret on preedit
 - Add function to return relative location of the caret (it's how it's handled on Linux).
 - Merge branch 'master' into new_ime
 - Merge branch 'master' into new_ime
 - Weird API
 - JavaFX does not currently supports surrouding
 - Remove dup event
 - Progress
 - Fix key typing
 - ... and 61 more: https://git.openjdk.org/jfx/compare/84aad81a...5d6bd04d

Changes: https://git.openjdk.org/jfx/pull/1080/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1080&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305418
  Stats: 525 lines in 16 files changed: 135 ins; 277 del; 113 mod
  Patch: https://git.openjdk.org/jfx/pull/1080.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1080/head:pull/1080

PR: https://git.openjdk.org/jfx/pull/1080