Implement activeViewlet and Related Context Keys

[tsmaeder]

Feature Description:

Some when-clause context keys are unimplemented in Theia (see https://code.visualstudio.com/api/references/when-clause-contexts#visible-view-container-when-clause-context).

We have definitions of the context keys in view-context-key-service, and the values are referenced in some places, but there seems to be no place where the value is actually set.

[tsmaeder]

Actually, there is an implementation, except for activeAuxiliary (see plugin-view-registry.ts#updateVisibleView(...)). However, an adopter reports this does not always work. Investigating.

[tsmaeder]

Further inquiry shows that the tracking only works for well known view container id's. For example, the activeViewlet === undefined when the plugin view is active instead of workbench.view.extensions. That means, of course, that view containers contributed by extensions don't work either.