Detection of mount points

Instead of implementing everything with static methods I would use an instance of `Mounts` as object to represent the mount points in the system. Then the code in `UnmanagedFilesInspector#inspect` can also written in a more object-oriented way by not having to create intermediate variables of results obtained from a procedure, but by accessing the object to get the information, which is needed. This would also obsoltete the `get_mount_points` function.