RFR: 8228363: ContextMenu.show with side=TOP does not work the first time in the presence of CSS [v2]

[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.

Robert Lichtenberger has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR.

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Changes:
  - all: https://git.openjdk.java.net/jfx/pull/381/files
  - new: https://git.openjdk.java.net/jfx/pull/381/files/48d6453a..d8bb41d1

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=381&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=381&range=00-01

  Stats: 144 lines in 2 files changed: 14 ins; 115 del; 15 mod
  Patch: https://git.openjdk.java.net/jfx/pull/381.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/381/head:pull/381

PR: https://git.openjdk.java.net/jfx/pull/381