Moved .up() and .down() methods from Modifier API to Modifier sequence
API (also renamed them to moveUp() and moveDown() ). Locating methods
which modify the "parent" structure in objects didn't seem consistent.
More Modifier API changes:
* add Blender.Modifier.Settings dict with constants for modifier types
* add mod.type attribute, which returns type of the Modifier
* add some internal consistency checks in ModSeq_remove