RFR: JDK-8313628: Column drag header, overlay and line are not correctly aligned

[Marius Hanl]

When a table has padding or the `layoutChildren` method inside the table skin is overridden (and x/y are modified), the drag drag header, column overlay and column line are not correctly aligned.

The reason is that the positions were calculated incorrectly.
- **Column overlay and column line**
Always calculate in the x and y from the table. The x and y variables contain the snapped insets (padding) and possible modifications from subclasses.
- **Drag header**
Calculate the drag x offset local bounds from the parent header (which is either the parent column header or the root header)
Before, the local bounds were calculated from the table, which will wrongly calculate in the padding.
We do not want to know the local bounds based of the whole table but of our header we are in.

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Commit messages:
 - JDK-8313628: Column drag header, overlay and line are not correctly aligned

Changes: https://git.openjdk.org/jfx/pull/1193/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1193&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313628
  Stats: 371 lines in 5 files changed: 336 ins; 7 del; 28 mod
  Patch: https://git.openjdk.org/jfx/pull/1193.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1193/head:pull/1193

PR: https://git.openjdk.org/jfx/pull/1193