Unified search - search in current app by default

As covered in #23835, searching for files may take a very long time, making file search quite unusable. But it also has the side effect, that searching for something else will create collateral damage by overwhelming the database with unnecessary, very expensive queries, which stack up with every new keystroke. While the tests in #23835 where performed with my test user, who has 4 shares to a very big storage, other users have an average of 25 shares, with 55 shares max. If that user types 5 keys in the search box, he will trigger 4x55 queries running 1.2s each...

As a quick fix, I removed SearchProvider registration for the files app.

Each search provider should be configurable (at least enable/disable systemwide)