RFR: 8338016: SplitMenuButton constructors should match MenuButton

[Andy Goryachev]

Adding SplitMenuButton constructors matching MenuButton:

- public SplitMenuButton(String text)
- public SplitMenuButton(String text, Node graphic)
- public SplitMenuButton(String text, Node graphic, MenuItem... items)

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Commit messages:
 - javadoc
 - 8338016: SplitMenuButton constructors should match MenuButton

Changes: https://git.openjdk.org/jfx/pull/1535/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1535&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338016
  Stats: 47 lines in 1 file changed: 45 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1535.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1535/head:pull/1535

PR: https://git.openjdk.org/jfx/pull/1535