Create Xapian::Query with Xapian::PostingSource

When a Xapian::Query uses a Xapian::PostingSource it should use the
relevant constructor that takes a Xapian::PostingSource object.
Fix XAPIAN_GLIB_AVAILABLE_IN for new XapianQuery symbols
Release reference to Xapian::PostingSource impl on finalize
Call constructed() parent method before initializing our own fields
Move heavy-lifting of slot property onto XapianValuePostingSource

It turns out that XapianValuePostingSource is not abstract. Move the
handling of the slot property in that class and make it instantiatable
too.
Add an example for simplesearch with pageRank