Investigate focus change and selection update `onModelRootChanged`

[planger]

See #1186 (reply in thread) for the origin of this issue.

We should investigate:

1. Why do we update the selection onModelRootChanged to an empty set? Imho this should be reconsidered if that's necessary / useful. See SelectionService.modelRootChanged?
2. Why does the update of the selection impact / steel the focus as reported in the discussion linked above?

[martin-fleck-at]

I opened a PR that no longer sends a selection update when we only change the root but all the selected elements stay the same. I think this was a remnant of some older code that we no longer need. This also prevents the Theia-GLSP-Selection-Forwarder from being triggered all the time. We still do the onModelRootChanged -> updateSelection as we want to re-calculate the selection based on the new root to see if the selection can still be applied. (the empty sets just means no change and we use the previous state). As for the focus change I can no longer reproduce this, changing the selection in Theia does not change the active widget or steal focus as far as I can tell.