Allow indenting multiple list items at the same time.

Previously, TinyMCE only allowed indenting a single list line at a
time. This change makes it so multiple lines may be selected, although
it maintains the restriction that the first list item cannot be
indented any further.

--HG--
extra : amend_source : 2c10b36bbebf6a242e4460b5d52a02bd9c860743