RFR: 8091153: Customize the Table Button Menu [v2]

[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.

Marius Hanl has updated the pull request incrementally with one additional commit since the last revision:

  JDK-8091153: Improve javadoc to match the CSR

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1135/files
  - new: https://git.openjdk.org/jfx/pull/1135/files/8fd5c719..aadbb432

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1135&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1135&range=00-01

  Stats: 6 lines in 1 file changed: 2 ins; 1 del; 3 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