Simplify type initilization

Before using a type in a GtkBuilder file, this type must be registered. A simple way to register a type is to state A_TYPE_B, which will call the type getter for type B of namespace A, registering it if it was not done before.

It is a pain to manually do that in C, and it's worse in higher level languages.

We should check how other widget libraries handle that (if at all), maybe having a hdy_init () function would help in a simple way.