RFR: 8272870: Add convenience factory methods for border and background [v4]

[Nir Lisker]

> Added convenience factory factory methods for Background and Border.

Nir Lisker has updated the pull request incrementally with one additional commit since the last revision:

  Added null tests and changed doc as per comments

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Changes:
  - all: https://git.openjdk.java.net/jfx/pull/610/files
  - new: https://git.openjdk.java.net/jfx/pull/610/files/345d759f..a550ff85

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=610&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=610&range=02-03

  Stats: 16 lines in 4 files changed: 14 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/610.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/610/head:pull/610

PR: https://git.openjdk.java.net/jfx/pull/610