RFR: JDK-8277396: [TESTBUG] In DefaultButtonModelCrashTest.java, frame is accessed from main thread [v2]

[Alexey Ivanov]

> It's a little cleanup: make sure `frame` is accessed from EDT only, remove unused variables and imports.

Alexey Ivanov has updated the pull request incrementally with three additional commits since the last revision:

 - Re-grouped UI creation code for clarity
 - Clarified test description
 - Add 1 sec delay after showing frame

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6455/files
  - new: https://git.openjdk.java.net/jdk/pull/6455/files/c637f86c..200909ad

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6455&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6455&range=00-01

  Stats: 10 lines in 1 file changed: 5 ins; 3 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6455.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6455/head:pull/6455

PR: https://git.openjdk.java.net/jdk/pull/6455