Device Backup: Fix VLC directory hiding in Files app + Exclude Media Library database

Why ? On iOS 11+, if we exclude Documents directory from backup, Files.app might hide VLC's folder.
Solution : apply isExcludedFromBackup recursively when the user changes the Backup media library setting.
Problem : depending on the device and the quantity of files, this can be really slow.

Solution : adopt strategies that speed up the process of exclusion

• On iOS 9 / 10 (usually old and slow devices), Files.app does not exist so we exclude Documents as before. One action, fast.
• On iOS 11+, we exclude the first level of files in Documents but not Documents itself. Files.app won't hide VLC but this will limit the amount of actions.

However, as older versions of VLC excluded the library on a per-file basis, these strategies can only be applied to exclusion.
Inclusion must be applied recursively to every files, to be sure that after an update, no file are left excluded.

On file add :

• Exclude or include any added file according to the Backup media library setting

On setting change :

	Exclude from backup	Include in backup
iOS 11+	Exclude Documents' top level files	Include recursively the Documents directory
iOS 9 / 10	Exclude Documents directory	Include recursively the Documents directory

On app start :

	Updating from the previous version	Updating from an older version
on iOS 11+, excluded from backup	reinclude Documents and exclude top-level files in Documents	do nothing
on iOS 11+, included in backup	reinclude Documents and its content recursively	reinclude Documents' content recursively
on iOS 9 / 10, excluded from backup	do nothing	exclude Documents
on iOS 9 / 10, included in backup	reinclude Documents and its content recursively	reinclude Documents' content recursively