RFR: 8338016: SplitMenuButton constructors should match MenuButton [v2]

[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)

Andy Goryachev has updated the pull request incrementally with one additional commit since the last revision:

  tests

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1535/files
  - new: https://git.openjdk.org/jfx/pull/1535/files/8aaa08b0..3f08be59

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1535&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1535&range=00-01

  Stats: 63 lines in 1 file changed: 62 ins; 0 del; 1 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