RFR: 8326734: text-decoration applied to <span> lost when mixed with <u> or <s> [v2]

[Alexey Ivanov]

> The value of the [`text-decoration`](https://www.w3.org/TR/REC-CSS1/#text-decoration) CSS property is not inherited correctly in Swing. If the `<span>` element is mixed with `<u>` or `<s>`, only the value from the `style` attribute of `<span>` is applied.
> 
> The fix to this issue is not as simple as that for the previous one in PR #17659, [JDK-8323801](https://bugs.openjdk.org/browse/JDK-8323801). Even in the seemingly simple case where `<u>` is followed by `<span style='text-decoration: line-through'>`, the situation is more complex because the styles are stored in `MuxingAttributeSet` in different elements of the array.
> 
> To resolve this problem, `CSS.Attribute.TEXT_DECORATION` is treated as a special case. Indeed, it is a special case: the values set to a single `text-decoration` property should be combined across the entire tree of nested HTML elements and their styles.
> 
> So, `MuxingAttributeSet` looks for `text-decoration` in the entire array and combines all the values.
> 
> The same way, `StyleSheet` also goes up the inheritance chain by combining the current value of `text-decoration` with that from `getResolveParent`.
> 
> The `ConvertSpanAction` combines the value of `text-decoration` of adjacent `<span>` elements.
> 
> Finally, `ConvertAction` and `CharacterAction` are refactored. The `ConvertAction` class duplicated the code from `CharacterAction`. Now `ConvertAction` extends `CharacterAction` and overrides a method to provide additional handling.
> 
> Thus, [JDK-8325620](https://bugs.openjdk.org/browse/JDK-8325620) is also resolved by this PR, the action used for `<b>`, `<i>`, `<u>` is `CharacterAction` as specified.

Alexey Ivanov has updated the pull request incrementally with two additional commits since the last revision:

 - Add 8326734 to HTMLUnderlineStrike test written for 8323801
 - Add tests for 'underline' and 'line-through'

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18550/files
  - new: https://git.openjdk.org/jdk/pull/18550/files/e9cf30b3..d20f8ce8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18550&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18550&range=00-01

  Stats: 265 lines in 3 files changed: 264 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18550.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18550/head:pull/18550

PR: https://git.openjdk.org/jdk/pull/18550