HTML facet for Stapler

[jglick]

PoC for jenkinsci/jenkins#21523.

• dynamically inject text into a page
• conditionally include a block
• repeat a block
• inject text into attribute values
• conditionally include attribute (such as checked)
• inject well-formed HTML subtree
• easy way to localize text (not just injecting a record field delegating to Messages)
• render recursive structure
• accept any POJO with the right methods in place of records, to permit lazy calculation
• access a contextual variable (perhaps as an annotated method parameter)
• include another HTML view
• optional view include (dispatchable object is null)
• optional view include (dispatchable object is not null but does not define view)
• set some contextual variables on include
• include a Jelly (or Groovy) view, defining it
• be included from a Jelly (or Groovy) view
• accept StaplerRequest and/or StaplerResponse from handler method
• throw HttpResponseException from handler method
• consume taglib
• define taglib
• view from nested class
• view from a subtype implementing abstract parts of a supertype
• supertype defines optional view, subtype may implement
• use a prefix like st: on active ids (no XML namespaces like Wicket)
• annotation processor validating that declared view resources exist (and no undeclared resources exist)
• annotation processor validating that st.* ids match record structure
• (perhaps) annotation processor that not just validates structure but actually generates code for a handler statically calling model methods (probably still parsing DOM at runtime)
• review content type

  stapler/html/src/test/resources/org/kohsuke/stapler/html/IncludedFromTest/center.xhtml

  Line 2 in 6a8257c

  <!-- TODO find a way to declare that this is an XHTML fragment, e.g. https://stackoverflow.com/a/734212/12916 -->

  stapler/html/src/main/java/org/kohsuke/stapler/html/impl/HtmlFacet.java

  Lines 80 to 81 in 6a8257c

  	// TODO allow *.html if it can be parsed without external deps
  	// or use *.xml?

  so that document fragments can either be statically validated, or validated by an annotation processor against the XHTML Schema
• verify *.xhtml reloading from hpi:run
• demo in core of Run/index loading allActions → InterruptedBuildAction/summary → CauseOfInterruption/UserInterruption/summary and also AbstractBuild/details
• demo in core of some (complex) config form
• demo of some plugin (like workflow-job) interacting with core