RFR: 8266966: Wrong CSS properties are applied to other nodes after fix for JDK-8204568

[Ambarish Rapte]

Issue:
When css file is added to a `Scene.getStylesheets()` of a Scene which has few controls in addition to a Label then css style of a different control gets applied to Label after Stage is hidden and shown. This is a regression of [JDK-8204568](https://bugs.openjdk.java.net/browse/JDK-8204568).

The sample program attached to JBS adds a Label, Button, CheckBox and another Label to Scene. The css style of the Button gets applied to second Label after Stage is hidden and shown.

Fix: Do not `recalculateRelativeSizeProperties()` when `resetToInitialValues()` is in progress.
After Stage is hidden and shown, the css style is reset to initial value in method `resetToInitialValues()`, which resets the font size of a Control to default font size. This reset results in execution of method `recalculateRelativeSizeProperties()` that was added as part of fix for JDK-8204568. `recalculateRelativeSizeProperties()` uses `getStyleMap()` to get `StyleMap` of the node. In this case `getStyleMap()` returns an incorrect `StyleMap` as the reset is still in progress. 

A comment is added in this fix to explain how the StyleMap is stored and retrieved.
Added a system test which fails before and passes after this fix.

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Commit messages:
 - do not recalculate when reseting css

Changes: https://git.openjdk.java.net/jfx/pull/504/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=504&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266966
  Stats: 130 lines in 3 files changed: 127 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/504.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/504/head:pull/504

PR: https://git.openjdk.java.net/jfx/pull/504