- compile against LWJGL 3.2.3 version
- restrict to the four BGFX demos (code & resources)
- add Gradle wrapper & build script
- use GraalVM native-image utility to produce native executable
  containing the four BGFX demos

the README.md changes are specific to this branch only.

- org.lwjgl.Version's getVersion() method is patched in native image
to retrieve the build number correctly from manifest entry
- system properties from the command line can be passed on to the
running demo in Gradle/Maven build scripts and in native image

to make it work in Linux.

Also, remove the ThreadLocalUtil's substitution by GraalVM,
as the resulting native image seems to work fine without it.