- refactored folder hierarchy for various components
(for future spliting out of viewer repo)
 * put all transforms related code into "transforms" folder
 * put all io related code into "io" folder
 * put all annotations related code into "annotations" folder
 * modified imports structure accordingly

- a bit more work on "scene" import/export:
 * bad news : it cannot works as it is, need rethinking
because of the dependencies between various parsers etc
 * also needs more thoughts on how to handle external resources
 etc etc freezing this for now