RFR: 8273657 : TextField: all text content must be selected initially

[Karthik P K]

The text was not getting selected on adding the `TextField` to the scene initially, subsequently removing and adding the `TextField` to the scene selects the entire text present in the `TextField`. 

Made changes in the `TextFieldBehavior` constructor to select the text on adding the `TextField`.

Added unit test to validate the fix

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Commit messages:
 - Fix textfield selection issue

Changes: https://git.openjdk.org/jfx/pull/1446/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1446&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8273657
  Stats: 25 lines in 2 files changed: 25 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1446.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1446/head:pull/1446

PR: https://git.openjdk.org/jfx/pull/1446