Support asynchronous computation of content assist proposals

[CrushaKRool]

Eclipse introduced a flag in 4.14 that lets code completion providers indicate whether they require to run in the UI thread or can be computed in the background, to provide a more fluent user experience when hitting the content assist key.

However, it seems like the Groovy Eclipse proposals are currently preventing this feature from being used in the IDE.

Perhaps it wouldn't take much to make the computation from this plugin async-compatible as well?

[eric-milles]

ready to test

[CrushaKRool]

Looks good to me. No longer prevents that option from being enabled in the settings and couldn't notice any obvious issues when opening Content Assist.