RFR: 8087673: [TableView] TableView and TreeTableView menu button overlaps columns when using a constrained resize policy.

[Jose Pereda]

The corner region is currently laid out with this assumption:

        // position the top-right rectangle (which sits above the scrollbar)

However, the vertical scrollbar is not always visible. Therefore, when that is 
the case, the corner region is laid out on top of the table column headers, 
overlapping any of their graphic or text content. In case of the last visible 
column header, it is not always possible to scroll horizontally to see the full 
content, either because of a constrained resize policy or because there is no 
more space left after the last column.

This PR fixes this issue by considering as padding for the last visible column 
header the presence of the corner region over it, only when the vertical 
scrollbar is not visible and the corner region is.

A couple of tests have been added to both TableViewTest and TreeTableViewTest.

Test 1: Before and after the changes :

<img width="312" alt="image" 
src="https://user-images.githubusercontent.com/2043230/187190928-efc29e38-8de2-4daf-8eff-a218d6fae181.png";>
<img width="312" alt="image" 
src="https://user-images.githubusercontent.com/2043230/187190991-deaa88b3-2f52-48bc-b3ec-107a5151f7b5.png";>

Test 2:  Before (sort arrow is there, but not visible) and after the changes:
<img width="412" alt="image" 
src="https://user-images.githubusercontent.com/2043230/187191310-fb348d8c-84fc-4254-a1a5-91811bf09b79.png";>
<img width="412" alt="image" 
src="https://user-images.githubusercontent.com/2043230/187191358-0d4937cb-b3b4-4530-9489-ff8cbf0c8443.png";>

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

Commit messages:
 - Include as padding the corner region over the last visible column header

Changes: https://git.openjdk.org/jfx/pull/886/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=886&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8087673
  Stats: 238 lines in 4 files changed: 230 ins; 2 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/886.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/886/head:pull/886

PR: https://git.openjdk.org/jfx/pull/886