glib: install properties before class_init finishes

Recently glib started to warn about the future deprecation of adding
interfaces after class_init, which we fixed here [1].

Now, a similar warning has started to happen for the addition of
properties, example:

(Nereid:23225): GLib-GObject-WARNING **: Attempt to add property __gtksharp_33_Banshee_Widgets_TileView::gtk-sharp-managed-instance after class was initialised

So we need to add them before class_init finishes too, and this
change makes it this way.

[1] https://github.com/mono/gtk-sharp/commit/9ff7ec8b2c6a1727f61601bd934d65a790f5adda