use rb_data_type_t instead rb_data_type_struct

For some reason the struct tag and typedef name differ and
nobody uses rb_data_type_struct.
get rid of INIT_TYPE() / initType()

A function to initialize rb_data_type_t variables is not needed.

Also fix a FIXME (WindowVX's name is now "Window") on the way.
Merge pull request #58 from cremno/mri-use-rb_data_type_t

MRI: use rb_data_type_t and initialize in definition