Instead of a generic function to filter GLists, implement a simpler
and clearer filter function for file lists, since it was the only
use of that function.
In this way the ownership of files and directories are clearer
since it always returns a new allocated nautilus file list, and also
it always uses nautilus_file_ref instead of the generic g_object_ref
to match what we do everywhere else in nautilus, so it's not confusing
when breaking at nautilus_file_ref/unref for ref counting debugging.

This change fixes multiple leaks on nautilus files catched by valgrind.