Before this commit folder with no direct comics (only subfolders) didn't have a cover image to be displayed. Now updating the children info is done recursively and subfolders are taken into account.
This affects the iOS client remote browser so far, but it is also needed for the future browser update (display folders as a grid instead of using EmptyFolderWidget)
Qt's database and query model requires that both the queries and the database
objects are out of scope before a database connection can safely be removed.
Solution: Properly encapsulate databases and queries in "{ }" and use a string
to cache the connection name for out-of-scope removal.
