RFR: 8204568: Relative CSS-Attributes don't work all time

[Ambarish Rapte]

Issue is that the size of properties that are relatively(`em`) sized is not computed correctly when the reference `-fx-font-size` is also specified relatively and is nested.

Fix is a slight variation of an earlier suggestion in [the PR](https://github.com/javafxports/openjdk-jfx/pull/94).

Fix is very specific to this scenario and did not show any side effect nor any test failure.

There are 12 new unit tests added along with fix:
- Two tests fail before and pass after this fix. These test verify the reported failing scenario.
sameRelativeFontSizeNestedParentTest
relativeFontSizeDeepNestedParentControlTest
- Two other tests fail both before and after this fix. They are not related to the fix. These two tests are ignored. I shall file new JBS issues to track these cases and update PR with the IDs added to @Ignore.
propertySizesRelativeToFontSizeOfControlTest
propertySizesRelativeToFontSizeOfParentTest
- Other 8 tests are sanity tests which pass both before and after this fix.

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Commit messages:
 - fix and test

Changes: https://git.openjdk.java.net/jfx/pull/397/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=397&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8204568
  Stats: 319 lines in 2 files changed: 317 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/397.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/397/head:pull/397

PR: https://git.openjdk.java.net/jfx/pull/397