label: Make attr_list_merge() use sane merging rules

label: Make attr_list_merge() use sane merging rules

Instead of "attribute with later start index wins, make sure the
attribute list that is merged from takes precedence. This now gives the
multiple attribute lists we use in the label an order:

1) gtk_label_set_attributes()
  These attributes override everything. It's what the function's there
  for after all.
2) markup of label
  Other user-specified attributes come next.
3) attributes for links
  When we apply custom attributes on parts of the text, we put them
  last. We don't want to mess with what the user does. Also, we change
  color and underline, so we usually have something to show.