RFR: 7148092: [macosx] When Alt+down arrow key is pressed, the combobox popup does not appear. [v6]

[Prasanta Sadhukhan]

> When comboBox have focus, pressing Alt+Down (or Up) should render the popup which is not happening for Aqua L&F.
> 
> Fix is to add toggleAction for Alt+Down/Up keys as has been done for [MetalLookAndFeel](https://github.com/openjdk/jdk/blob/master/src/java.desktop/share/classes/javax/swing/plaf/metal/MetalLookAndFeel.java#L912)
> 
> closed problemlisted test is used to check the fix.

Prasanta Sadhukhan has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains six commits:

 - Merge master
 - Test fix
 - Add test
 - Use camelCase
 - Fix as seen in native combobox
 - 7148092: [macosx] When Alt+down arrow key is pressed, the combobox popup does not appear.

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Changes: https://git.openjdk.org/jdk/pull/10014/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10014&range=05
  Stats: 128 lines in 3 files changed: 128 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10014.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10014/head:pull/10014

PR: https://git.openjdk.org/jdk/pull/10014