RFR: 8290399: [macos] Aqua LAF does not fire an action event if combo box menu is displayed

[Prasanta Sadhukhan]

When the user types Enter in an editable combo box, Aqua LAF fails to fire an action event if the menu is displayed.
It seems actionEvent is fired when popup menu is not visible. 
Fix is to make sure ActionEvent is fired irrespective of popup visiblity as is done in other L&F 
(for ex, in MetalL&F it calls "ENTER", JTextField.notifyAction, where it calls postActionEvent irrespectively)

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Commit messages:
 - 8290399: [macos] Aqua LAF does not fire an action event if combo box menu is displayed
 - 8290399: [macos] Aqua LAF does not fire an action event if combo box menu is displayed

Changes: https://git.openjdk.org/jdk/pull/9551/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9551&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290399
  Stats: 94 lines in 2 files changed: 92 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9551.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9551/head:pull/9551

PR: https://git.openjdk.org/jdk/pull/9551