RFR: 8234315: GTK LAF does not gray out disabled JMenu

[Abhishek Kumar]

Disabled JMenu foreground color was not grayed out in GTK LAF for version 3. The issue didn't occur in GTK-2. 
After analysis it has been found that the JMenu widget type was changed to JMenuBar and the color value returned for the foreground color in disabled state was close to black color (RGB 0,0,0) for menubar which is not differentiable from enabled state foreground color.

As a fix for this problem, if the menubar is in disabled state then the widget is changed to JMenu and color value returned for disabled menu is gray color.

An automated test case has been added and checked in CI, link is added in JBS.

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Commit messages:
 - Whitespace fix
 - Disabled JMenu foreground color grayed out fix

Changes: https://git.openjdk.org/jdk/pull/10176/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10176&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8234315
  Stats: 130 lines in 2 files changed: 130 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10176.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10176/head:pull/10176

PR: https://git.openjdk.org/jdk/pull/10176