RFR: 7902924: Add build script for jtreg plugin [v7]

[Maurizio Cimadamore]

> This patch adds support for building the jtreg plugin with gradle.
> 
> This is a much more stable way to build the plugin, as the IntelliJ build dependency is automatically downloaded by gradle.
> 
> The build relies on the IntelliJ support for gradle-ized plugins:
> 
> https://github.com/JetBrains/gradle-intellij-plugin
> 
> Which is officially supported by JetBrains, see:
> 
> https://plugins.jetbrains.com/docs/intellij/gradle-build-system.html
> 
> 
> The only thing that needs to be done by developers before firing up a gradle build, is to specify the path of a jtreg root in the gradle.properties file. Everything else should work out of the box.
> 
> I have also updated build instructions to reflect the new build system.
> 
> Please let me know if this is an improvement over the build steps we had before (I think it is, but I have not tested this extensively).

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Simplified instruction for installing plugin (e.g. use Help -> Find Action)

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

Changes:
  - all: https://git.openjdk.java.net/jtreg/pull/9/files
  - new: https://git.openjdk.java.net/jtreg/pull/9/files/692eaeec..ee0b481c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jtreg&pr=9&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jtreg&pr=9&range=05-06

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/9.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/9/head:pull/9

PR: https://git.openjdk.java.net/jtreg/pull/9