RFR: 8091153: Customize the Table Button Menu

[Marius Hanl]

This PR implements a way to override the table column menu. 
When the `cornerRegion` is pressed, it will now call the `showColumnMenu` method. This new method is protected and therefore can be overidden by developers. If not overridden, the known default column 'ContextMenu' is created and shown.

This PR also resolves [JDK-8091419 ](https://bugs.openjdk.org/browse/JDK-8091419) (The method `showColumnMenu` can be overridden and made public now)

This PR also helps with [JDK-8092148](https://bugs.openjdk.org/browse/JDK-8092148), but does not address all the points mentioned in the ticket. 
Now that everyone can provide their own implementation, we can think of treating the current implementation of the table column menu as an [MVP](https://en.wikipedia.org/wiki/Minimum_viable_product) and any other requested feature/idea should be fulfilled by providing your own implementaion of the menu.

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Commit messages:
 - JDK-8091153: Implement ability to override and therefore customize the column menu

Changes: https://git.openjdk.org/jfx/pull/1135/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1135&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8091153
  Stats: 123 lines in 3 files changed: 109 ins; 12 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1135.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1135/head:pull/1135

PR: https://git.openjdk.org/jfx/pull/1135