Use default_init instead of base_init when registering interfaces

Use default_init instead of base_init when registering interfaces

Using base_init() is overly complex, requires multi-initialization
guards, and in the case of vala generated code these are currently
not thread safe.

The recommended approach is to use default_init() to initialize
interfaces. It is called only once, solves thread safety issues.

https://bugzilla.gnome.org/show_bug.cgi?id=699550