Version Control With Subversion Source

My first pass across chapter 4. There's still much to be done, but
I think the basic framework is correct now, at least.

* src/en/book/ch-advanced-topics.xml
Gut the Properties section, reorganizing things to be topically
keyed instead of trying to read like an exhausing property
reference.

* src/en/book/ch-basic-usage.xml
Move the first mention of symbolic links into this chapter.

* src/en/book/ch-reference.xml
* src/en/book/app-svn-for-cvs-users.xml
Update some xref IDs.