writable-database: don't forget to call G_ADD_PRIVATE()

Or we'll segfault while initializing the object.
writable-database: use correct macros for type checks
test: add an unit test for XapianWritableDatabase

To test the bug we just fixed.
xapian-glib: remove duplicate type macro definitions
Merge pull request #19 from endlessm/sdk/2507

Sdk/2507