RFR: 8090547: Allow for transparent backgrounds in WebView [v7]

[Jose Pereda]

> Currently, `WebPage` has already a public `setBackgroundColor()` method, but the class is not public. Therefore, public API is needed in `WebView` to allow developers access to it.
> 
> In line with the `fontSmoothingType` property, this PR provides public support for setting the background color of a WebPage, by adding a `pageFill` property, and a CSR is required.
> 
> The color for the background, that can be opaque, transparent or with any level of opacity, can be set via code or via CSS using `-fx-page-fill`.
> 
> Unit tests and a system test are provided.

Jose Pereda has updated the pull request incrementally with one additional commit since the last revision:

  Address feedback from reviewers

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Changes:
  - all: https://git.openjdk.java.net/jfx/pull/563/files
  - new: https://git.openjdk.java.net/jfx/pull/563/files/941ba714..02061b9e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=563&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=563&range=05-06

  Stats: 61 lines in 4 files changed: 9 ins; 35 del; 17 mod
  Patch: https://git.openjdk.java.net/jfx/pull/563.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/563/head:pull/563

PR: https://git.openjdk.java.net/jfx/pull/563