RFR: JDK-8261665: Clean up naming of StringContent and FixedStringContent [v2]

[Jonathan Gibbons]

> Relatively simple update to clean up some of the old naming in the `html.markup` world.
> 
> `FixedStringContent` is renamed to just `Text` and a new factory method `Text.of(CharSequence)` introduced.
> 
> `StringContent` is renamed to `TextBuilder`.
> 
> `StringContent` was the original type and is widely used; `FixedStringContent` was added much later, along with `Contents` to support shared reusable tree nodes.  But most uses of `StringContent` do not need mutability, and most can be replaced with the lighter-weight `Text` class.  This change is done, just leaving `textBuilder` to be used in `ContentBuilder` and `HtmlTree` which support the `add(CharSequence)` method.

Jonathan Gibbons has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - Merge remote-tracking branch 'upstream/master' into text-rename
   
   # Conflicts:
   #	src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/formats/html/ClassUseWriter.java
   #	src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/formats/html/ClassWriterImpl.java
   #	src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/formats/html/Contents.java
   #	src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/formats/html/HtmlDocletWriter.java
   #	src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/formats/html/ModuleWriterImpl.java
   #	src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/formats/html/PackageUseWriter.java
   #	src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/formats/html/PackageWriterImpl.java
   #	src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/formats/html/TagletWriterImpl.java
   #	src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/formats/html/TreeWriter.java
 - JDK-8261665: Clean up naming of StringContent and FixedStringContent

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Changes: https://git.openjdk.java.net/jdk/pull/2556/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2556&range=01
  Stats: 574 lines in 41 files changed: 200 ins; 206 del; 168 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2556.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2556/head:pull/2556

PR: https://git.openjdk.java.net/jdk/pull/2556