Issue 483

[fvgh]

Proposed fix for #483.
The changes remove duplication in the Spotless build scripts.
_ext is build with Gradle 6 like the rest of the Spotless projects.
The _ext projects become sub-projects of the Spotless root project, but are deactivated by default.
The _ext projects are activated by root project properties which allow activation by command line as well as per user gradle.properties.

The changes additionally contain:

• Work-around of WTP build (latest EFS changes broke the build, fix will be applied with next WTP update). Due to the dependency lock, the current WTP releases are not affected by the EFS change.
• Fix of JDT tests which are broken since Eclipse 4.13 support #480

[fvgh]

@nedtwigg Please have a look at the mechanism to activate _ext projects. I changed the approach, since I did not wanted to fumble with the Eclipse start-up. I think the user properties is an adequate location to store such user specific modification to the build process.
The p2 conversion still is sometimes buggy (clean is required). But the clean&build for dedicated sub-projects works fine.

[nedtwigg]

This looks excellent to me. It looks to me like there are only two small behavior changes, which I noted as comments below. Neither is a problem, just wanted to call attention to them.

One other small thing is that Goomph had some deprecations in the latest version 3.21.0 - main thing is renaming com.diffplug.gradle.blah to com.diffplug.blah. Might want to incorporate that version bump into this PR.

[fvgh]

@nedtwigg Please have a quick look at last comments and changes. I would prefer a squashed merge, since all changes are just related to build clean-up.

[nedtwigg]

Looks great, I'll let you make a call re: UTF-8 above, then you can squash and merge :)

[fvgh]

@nedtwigg Sorry, was sleeping. Missed that the PR title is used as comment. Think the only way back is an amend and forced push, isn't it?

[nedtwigg]

Correct, but I think it's fine :)