imprv: always use __init__ instead of __new__

[bryanforbes]

For structs and boxed objects, PyGObject uses __new__ for the constructor named "new" under certain cases along with a dummy __init__ that does nothing. In addition, some GObject.Object overrides use __new__ (ex. Pango.Layout) and some struct and boxed overrides use both __new__ and __init__ or one or the other. Finally, PyGObject incorrectly uses Self in their type annotations. The following changes have been made to handle all of these scenarios:

• __new__ and __init__ handling has been moved to ClassInfo.__build_init()
• For structs and boxed, __new__ is used to build __init__ when either __new__ is the same as the "new" method from __info__ or __new__ exists directly on the class; otherwise, __init__ is used
• For GObject.Object subclasses, __new__ is used when it exists directly on the class; otherwise, __init__ is built from the writable, construct, and construct-only props
• Writing all constructors as __init__ eliminates the improper use of Self that comes from PyGObject

[bryanforbes]

@xclaesse this fixes the errors about __new__ and Self being used improperly