RFR: 8314683: TextArea: scroll bar size and content padding

[Andy Goryachev]

Changing certain ScrollBar, ScrollPane, and TextArea sizes from `em` to `px` in modena.css to make them independent of the font size *in that control*.

After the change, the UI still looks good which can be tested by scaling default font size in a fairly wide range:


.root { -fx-font-size:100%; }


Using CSS Playground tool in the Monkey Tester at 50% and 200% font size:

![Screenshot 2024-05-31 at 14 48 41](https://github.com/openjdk/jfx/assets/107069028/c41e1500-ccfe-4aa6-b47a-42571f60b335)

![Screenshot 2024-05-31 at 14 49 34](https://github.com/openjdk/jfx/assets/107069028/78270097-4ce3-4c3e-8600-aba8d368495c)

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Commit messages:
 - 8314683 TextArea: scroll bar size and content padding

Changes: https://git.openjdk.org/jfx/pull/1469/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1469&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314683
  Stats: 8 lines in 1 file changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jfx/pull/1469.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1469/head:pull/1469

PR: https://git.openjdk.org/jfx/pull/1469