Add support for async completions in 4.15

[gayanper]

Marked the SpringBootJavaCompletionProposalComputer as requiresUIThread
= false so that the completions can be run asyncly.

This will make the new java async support to work even when STS4 tooling is installed.

[kdvolder]

How do we know that this works?

[gayanper]

https://bugs.eclipse.org/bugs/show_bug.cgi?id=531061

We need to test with 4.15 M1. I can test the nightly build when this change is available. When will it be available @BoykoAlex ?

[martinlippert]

@gayanper You can get the latest CI builds from here: http://dist.springsource.com/snapshot/STS4/nightly-distributions.html (full distribution builds as well as p2 repo URLs)

[gayanper]

I can still see this warning

From where does this completion comes from ?

[BoykoAlex]

It comes from spring-ide project: https://github.com/spring-projects/spring-ide

[gayanper]

Can we remove that from the installation without effecting much of sts4 functionality? Or should we fix it as well ?

[kdvolder]

When I am asking "how do we know this works" it is not so much about whether that flag itself works. I'm questioning whether you can just make the affected completion providers run on non-ui thread without throwing 'invalid thread' exceptions and/or misbehaving in any other unexpected ways.

There is actually quite a lot of Eclipse api that just throws invaldid thread excpeption when callled from non-ui thread.

So yeah, testing thoroughly whether the changed completion providers still work is important.