RFR: 8228363: ContextMenu.show with side=TOP does not work the first time in the presence of CSS

[Robert Lichtenberger]

By using the anchor location facility of PopupWindows we can avoid miscalculation of the
menu's height entirely.
This fix also cleans up some documentation issues.
This fix introduces tests that check the correct positioning (test_position_)
test_position_withCSS reproduces the problem that is fixed with this patch.
The other test_position_ cases serve as "proof" that no regressions are introduces.
They work before and after the fix is introduced.

-------------

Commit messages:
 - 8228363: ContextMenu.show with side=TOP does not work the first time in the presence of CSS
 - Merge pull request #1 from openjdk/master
 - Merge remote-tracking branch 'upstream/master'
 - Merge remote-tracking branch 'upstream/master'
 - Merge remote-tracking branch 'upstream/master'
 - Merge remote-tracking branch 'upstream/master'
 - Merge remote-tracking branch 'upstream/master'
 - Merge remote-tracking branch 'upstream/master'
 - 8232524: Test cleanup: terminate background thread upon failure.
 - 8232524: SynchronizedObservableMap cannot be be protected for copying/iterating
 - ... and 1 more: https://git.openjdk.java.net/jfx/compare/d8bb41d1...48d6453a

Changes: https://git.openjdk.java.net/jfx/pull/383/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=383&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8228363
  Stats: 144 lines in 2 files changed: 115 ins; 14 del; 15 mod
  Patch: https://git.openjdk.java.net/jfx/pull/383.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/383/head:pull/383

PR: https://git.openjdk.java.net/jfx/pull/383