RFR: JDK-8310118: Resource files should be moved to appropriate directories

[Hannes Wallnöfer]

Please review a change to rename the directories previoiusly called `resources` and `script-dir` in `javadoc`-generated documentation to `resource-files` and `script-files`, and consistently moving all files that are not related to the documented code into these directories. This includes scripts and stylesheets added via command line options.

In the process I cleaned up the code which copies scripts and resources. I added a new private `HtmlDoclet.copyResource` method which takes care of resolving resources relative to the `doclet.formats.html` or `doclet.toolkit` packages and determines whether a resource needs localization based on the file extension of the source resource. The new code will also throw an exception if the source resource does not exist, while previously `DocFile.copyResource` returned silently. 

Another unrelated cleanup was to remove a bunch of jQuery-UI image files in a resource subdirectory that were not used anymore.

In addition to make sure all tests pass I did extensive manual testing of the generated docs to make sure all scripts and resources are loaded correctly. This includes the `@import` statement for `resources/fonts/dejavu.css` in the default stylesheet, which is now changed to `fonts/dejavu.css` assuming the `fonts` directory will now be located in `resource-files`.

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Commit messages:
 - JDK-8310118: Resource files should be moved to appropriate directories

Changes: https://git.openjdk.org/jdk/pull/15180/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15180&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310118
  Stats: 255 lines in 39 files changed: 64 ins; 79 del; 112 mod
  Patch: https://git.openjdk.org/jdk/pull/15180.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15180/head:pull/15180

PR: https://git.openjdk.org/jdk/pull/15180