RFR: 8353668: Rename internal c.s.javafx.text.TextLine class

[Andy Goryachev]

This minor change renames an internal `com.sun.javafx.text.TextLine` to `PrismTextLine`.
This class implements `com.sun.javafx.scene.text.TextLine` interface, also internal, for the purpose of reducing confusion and avoiding FQCN.

This change is completely transparent to the application developers since no public APIs is impacted.

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Commit messages:
 - cleanup
 - prism text line

Changes: https://git.openjdk.org/jfx/pull/1760/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1760&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353668
  Stats: 211 lines in 4 files changed: 96 ins; 96 del; 19 mod
  Patch: https://git.openjdk.org/jfx/pull/1760.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1760/head:pull/1760

PR: https://git.openjdk.org/jfx/pull/1760