ML: keep root storage after ML reset

Instead of re-adding the default internal storage as an ML root, save previous roots, reset the db, and re-add these roots.

Fixes #3122 (closed)

added Media Indexing & Backend label

approved this merge request

added 53 commits

• 47a98189...ededfb8f - 52 commits from branch videolan:master
• b9ec08c8 - ML: keep root storage after ML reset

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enabled an automatic merge when all merge checks for b9ec08c8 pass

merged

mentioned in merge request !2124 (merged)