Check for the case that a new file to be added to a .qmlproject
is also automatically covered e.g. by a wildcard filter. In this
case we don't have to do anything.
Will be passed to qmlviewer with the "-L" option. Storing this in
the .qmlproject file format itself (and not in the .user file) is useful
in case the libraries are relative/part of the checkout that is shared
between users/computers.
