Associations manager: Test if file handle is valid before reading.
Copy operation: Raise exception if file handle is invalid.
Plugins: File list: Simplify code.
Plugins: File list: Handle URI paths in LocalProvider.
Plugin base: Item list: Additional checks for providers.

Created method for checking if provider exists for specified path. This
is used to check if archive provider is already created for specified
path. Additionally we now destroy providers when they are no longer
needed. This provides archive providers with opportunity to clean up
before being destroyed.
Plugins: File list: Check if archive provider exists before creating.