blendfile.py: fix/add some 'basic data' reading (mostly arrays of...

blendfile.py: fix/add some 'basic data' reading (mostly arrays of ints/floats/...), add utils to recursively get all items of a struct.

New `BlendFileBlock.items_recursive_iter()` will yield all valid key/values pair of a given struct,
including sub ones (e.g. 'id.next', 'id.name', etc.).