[markers] add a way to prune stale markers

[JanKoehnlein]

I am switching between different Theia apps a lot. On each start, Theia reads the persisted markers from local storage. In my case these often refer to a completely different workspace and Theia app, so the neither the referred files nor the marker types exist in the current instance. The only way I found to get rid of the stale markers is to clear the local storage in the developer tools of Chrome. This should be improved.

[svenefftinge]

The root cause for this is #904

[vince-fugnitto]

fixed since markers are no longer kept in local storage #2316