RFR: 8231644: TreeTableView Regression: Indentation wrong using Label as column content type [v3]

[Marius Hanl]

> This PR fixes a long standing issue with the TreeTableView indentation.
> 
> ![image](https://user-images.githubusercontent.com/66004280/124681647-473e7380-dec9-11eb-906d-4228fc39cbf9.png)
> 
> In short:
> **TreeTableCellSkin** overrides **leftLabelPadding()** to calculate the indentation (the result of this method is added to **x**).
> While this is fine, this method is not always called (by **LabeledSkinBase#layoutLabelInArea**), e.g. when no text is set. 
> So when a TreeTableCell only sets a graphic (e.g. via **setGraphic()** in **updateItem()**), the indentation will be messed up.
> 
> Fixed this by adding the calculated indentation to **x** before we call **layoutChildren()**.
> 
> We also need/should adjust every other location where `leftLabelPadding()` was used:
> - **computePrefHeight** -> prefWidth() is always called with -1, so nothing got broken by not overriding this, but we should do it of course to be accurate in case we do one day.
> - **computePrefWidth** -> this is needed for auto sizing. I saw that it was slightly off without, so this 100% needed.
> - **computeMinWidth** -> the min width of a cell is not used, so nothing got broken by not overriding this but same as in computePrefHeight(), we should comply with the specs.
> - **layoutChildren** -> I saw a slight off sizing if the indentation is not subtracted to the width.
> 
> As a result of this, all method do effectively the same as they did with an overridden `leftLabelPadding()` (just earlier/later).
> Note: I also added some tests which pass before and pass after.

Marius Hanl has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:

 - Merge branch 'master' of https://github.com/openjdk/jfx into 8231644-indentation
   
   
 Conflicts:
   
	modules/javafx.controls/src/main/java/javafx/scene/control/skin/TreeTableCellSkin.java
 - calculated indentation in every method now which was previously done via leftLabelPadding
 - 8231644: fixed wrong indentation for tree cells with graphic only

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Changes: https://git.openjdk.java.net/jfx/pull/568/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=568&range=02
  Stats: 211 lines in 2 files changed: 197 ins; 0 del; 14 mod
  Patch: https://git.openjdk.java.net/jfx/pull/568.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/568/head:pull/568

PR: https://git.openjdk.java.net/jfx/pull/568