RFR: 8089913: CSS styling problem with Slider pseudo classes

[mstr2]

The Slider control does not have the ":horizontal" CSS pseudo-class set by default. The pseudo-class is only set once the "orientation" property is changed. This PR fixes that.

-------------

Commit messages:
 - 8089913: CSS styling problem with Slider pseudo classes
 - Failing test

Changes: https://git.openjdk.java.net/jfx/pull/413/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=413&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8089913
  Stats: 9 lines in 2 files changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/413.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/413/head:pull/413

PR: https://git.openjdk.java.net/jfx/pull/413