RFR: 8218826: TableRowSkinBase: horizontal layout broken if row has padding

[Marius Hanl]

This PR fixes a problem, where the layout is broken when a `(Tree)TableRow` has padding.
As also mentioned in the ticket, the `layoutChildren` method in `TableRowSkinBase` is implemented wrong.

The `layoutChildren` method is responsible for layouting all the `(Tree)TableCells`. 
When the row has padding, it is subtracted on every `(Tree)TableCell` - which is wrong.

Instead the `x` and `y` from `layoutChildren` should be used. (E.g. if `x` is 10, then only the first cell should start at 10 and the other cells follow as usual)
Also the `compute...` methods needs to add the padding as well.

**Example:**
_Row padding left right 0:_ 
[Cell1][Cell2][Cell3]
_Row padding left right 10:_ 
[    10    ][Cell1][Cell2][Cell3][    10    ]
_Same for top bottom._

When a `fixedCellSize` is set, the padding is currently ignored (also after this PR).
Therefore, `y` in the `layoutChildren` method is set to 0 for `fixedCellSize`.

This may can be discussed in the mailing list (Could be a follow up). To support padding also when a `fixedCellSize` is set, the `compute...` methods needs to also add the padding when a `fixedCellSize` is set (in the `if` clauses) and the `VirtualFlow` needs to add the padding to every row instead of using the `fixedCellSize` directly.

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Commit messages:
 - 8218826: fixed horizontal layout broken if row has padding

Changes: https://git.openjdk.java.net/jfx/pull/800/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=800&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8218826
  Stats: 439 lines in 3 files changed: 417 ins; 10 del; 12 mod
  Patch: https://git.openjdk.java.net/jfx/pull/800.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/800/head:pull/800

PR: https://git.openjdk.java.net/jfx/pull/800