Improve registration of default virtual navigations for easier override

Default coverage and subjects virtual navigations are not easy to override with Studio as they wait for the runtime to be started to be deployed => as Studio extensions also wait for the runtime to be started, order of deployment between the two bundles is not deterministic.

Workaround is to make Studio extensions wait for this specific contribution too, but it would be better to lazy-load virtual navigations, by performing checks done at registration later (when the tree is displayed for instance), so that order of deployment is easier to manage.