RFR: 8323706: Remove SimpleSelector and CompoundSelector classes [v7]

[John Hendrikx]

> Moves `SimpleSelector` and `CompoundSelector` to internal packages.
> 
> This can be done with only a minor API break, as `SimpleSelector` and `CompoundSelector` were public before.  However, these classes could not be constructed by 3rd parties.  The only way to access them was by doing a cast (generally they're accessed via `Selector` not by their sub types).  The reason they were public at all was because the CSS engine needs to be able to access them from internal packages.
> 
> This change fixes a mistake (or possibly something that couldn't be modelled at the time) when the CSS API was first made public. The intention was always to have a `Selector` interface/abstract class, with private implementations (`SimpleSelector` and `CompoundSelector`).
> 
> This PR as said has a small API break.  The other changes are (AFAICS) source and binary compatible:
> 
> - Made `Selector` `sealed` only permitting `SimpleSelector` and `CompoundSelector` -- as `Selector` had a package private constructor, there are no concerns with pre-existing subclasses
> - `Selector` has a few more methods that are now `protected` -- given that the class is now sealed, these modified methods are not accessible (they may still require rudimentary documentation I suppose)
> - `Selector` now has a `public` default constructor -- as the class is sealed, it is inaccessible
> - `SimpleSelector` and `CompoundSelector` have a few more `public` methods, but they're internal now, so it is irrelevant
> - `createMatch` was implemented directly in `Selector` to avoid having to expose package private fields in `Match` for use by `CompoundSelector`
> - No need anymore for the `SimpleSelectorShim`

John Hendrikx has updated the pull request incrementally with one additional commit since the last revision:

  Fix bug

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1333/files
  - new: https://git.openjdk.org/jfx/pull/1333/files/7d77e43e..39c94408

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1333&range=06
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1333&range=05-06

  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1333.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1333/head:pull/1333

PR: https://git.openjdk.org/jfx/pull/1333