Improve emitted DOM for indented lists

The editor currently emits overly complex DOM fragments to represent an indent list. This can easily provoke OOMEs when performing a PDF export of the page containing such a fragment.

E.g. if you insert an (ordered) list in the editor, add and entry and indented it via tab the output will be a nested (ordered) list.

It is enough to place 22 tabs on the list item to cause an OOME if you export the resulting page as PDF.

My test setup was a vanilla 5.2.2 instance against HSQLDB with -Xmx1G -XX:MaxPermSize=256M.

Server version: Apache Tomcat/6.0.35
Server built:   Dec 8 2012 11:07:24
Server number:  6.0.35.0
OS Name:        Linux
OS Version:     3.8.0-19-generic
Architecture:   amd64
JVM Version:    1.7.0_25-b15
JVM Vendor:     Oracle Corporation

Now I understand that this is really a problem of the PDF export (for which I'm going to raise a separate issue), but I believe the editor should make it an effort to use CSS instructions instead of DOM elements to reach the desired effect.