Glib::PropertyBase: Make custom properties more flexible

* glib/glibmm/property.[h|cc]: Relax the requirements on Property<>.
Now custom properties don't have to be direct data menbers of the containing
type. Instead, all objects of a class must instantiate the same properties
in the same order. This patch is an improved version of a patch supplied by
Povilas Kanapickas <povilas@radix.lt>. Bug #731484.