<Swing Dev> RFR: 8253016: Box.Filler components should be unfocusable by default

[Prasanta Sadhukhan]

Please review a fix for an issue where it is seen that even though Box.Filler components are invisible components that
are created by Box methods to return struts and glue, they ( Box.Filler instances) are focusable by default. As a
consequence, they become members of the implicitly created focus traversal policies, which means that their existence
as components can be seen by users.

Proposed fix is to make this Filler components unfocusable by default.
All automated headful tests and JCK tests are unaffected by these.

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Commit messages:
 - Modified testcase
 - Modified testcase
 - Modified testcase
 - Added testcase
 - 8253016: Box.Filler components shoule be unfocusable by default

Changes: https://git.openjdk.java.net/jdk/pull/149/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=149&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253016
  Stats: 85 lines in 2 files changed: 85 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/149.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/149/head:pull/149

PR: https://git.openjdk.java.net/jdk/pull/149