RFR: 8090547: Allow for transparent backgrounds in WebView

[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.

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Commit messages:
 - Support webView transparency (#1)

Changes: https://git.openjdk.java.net/jfx/pull/563/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=563&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8090547
  Stats: 482 lines in 8 files changed: 468 ins; 1 del; 13 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