[new validation engine] take preference changes into account beyond open files

[martinlippert]

The new validation engine (which runs as part of the overall indexing) doesn't react to preference changes yet, which results in diagnostics being published with a certain severity remain unchanged (and don't get re-published) when the user changes the severity of a certain validation.

This even happens across restarts due to the diagnostics being cached. Opening the file updates the diagnostics, but the mechanism should be improved to automatically update corresponding diagnostics if the severity changes for all files.

(follow-up action around new validation engine, based on work done for #1068)

[martinlippert]

Fixed with a93a42f