RFR: 8277122: SplitPane divider drag can hang the layout

[Marius Hanl]

When a divider is moved via drag or code it will call **requestLayout()** for the **SplitPane**.
While this is fine, it is also called when the **layoutChildren(..)** method is repositioning the divider.

This makes no sense since we are currently layouting everything, so we don't need to request it again (-> We are doing it right now).
After positioning the dividers the **SplitPane** content is layouted (based of the may new positioned dividers).

This PR fixes this by not requesting layout when we are currently doing it (and thus repositioning the dividers as part of it).

Note: I also fixed a simple typo of a private method in SplitPaneSkin:
initializeDivderEventHandlers -> initializeDiv**i**derEventHandlers

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Commit messages:
 - 8277122: SplitPane divider drag can hang the layout

Changes: https://git.openjdk.java.net/jfx/pull/669/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=669&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277122
  Stats: 61 lines in 2 files changed: 54 ins; 1 del; 6 mod
  Patch: https://git.openjdk.java.net/jfx/pull/669.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/669/head:pull/669

PR: https://git.openjdk.java.net/jfx/pull/669