Fix #1490: Allow page labels on loose columns.

Fix #1490: Allow page labels on loose columns.

Don't discard labels set on columns which are currently pruned or fall on
an empty barline.

* input/regression (page-label-loose-column.ly)

  new regtest, checks both unbreakable mid-line and empty barline labels

* lily/include/system.hh:

  add collect_labels ()

* lily/paper-column.cc (is_used):

  return true if 'labels is set

* lily/spacing-determine-loose-columns.cc (prune_loose_columns):

  set 'maybe-loose on columns which only contain page-labels

* lily/system.cc (break_into_pieces, collect_labels):

  collect labels from loose columns
  move label collection to separate method