This PR fixes a long standing issue with the TreeTableView indentation. 
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 indentaation will be messed up. Fixed this by adding the calculated indentation to **x** before we call **layoutChildren()**. -> We basically add the indentation to **x** when we call **layoutChildren()** instead of letting **layoutLabelInArea()** do it sometimes via **leftLabelPadding()**. (which is called directly by **layoutChildren()**) Note: I also added some tests which pass before and pass after. ------------- Commit messages: - 8231644: fixed wrong indentation for tree cells with graphic only Changes: https://git.openjdk.java.net/jfx/pull/568/files Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=568&range=00 Issue: https://bugs.openjdk.java.net/browse/JDK-8231644 Stats: 183 lines in 2 files changed: 168 ins; 0 del; 15 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
