Add API to check for extensions

[Michal Hruby]

Currently there's no easy way to determine if there is a zg-extension present or not (for example the FTS extension). It'd be nice if there was simple method which would do that.

[Mikkel Kamstrup Erlandsen (kamstrup)]

I started working on this but I am blocked on a bug in libeggdbus... :-S

[Mikkel Kamstrup Erlandsen (kamstrup)]

Just for the record. Here's what I get when running the included C example:

$ gcc $(pkg-config --libs --cflags zeitgeist-1.0) check-fts.c
kamstrup@hardback:~/Canonical/libzeitgeist/MASTER/examples$ ./a.out

(process:16591): GLib-GObject-WARNING **: invalid cast from `_EggDBusIntrospectableProxy' to `EggDBusObjectProxy'

** (process:16591): CRITICAL **: egg_dbus_object_proxy_query_interface: assertion `EGG_DBUS_IS_OBJECT_PROXY (object_proxy)' failed

** (process:16591): CRITICAL **: egg_dbus_introspectable_introspect_finish: assertion `EGG_DBUS_IS_INTROSPECTABLE (instance) && EGG_DBUS_IS_INTERFACE_PROXY (instance)' failed

(process:16591): GLib-GIO-CRITICAL **: g_simple_async_result_set_from_error: assertion `error != NULL' failed

(process:16591): GLib-CRITICAL **: g_error_free: assertion `error != NULL' failed
** (process:16591): DEBUG: 000000
Segmentation fault