Improve object saving and loading

* move ships and components to a separate namespace
* add versions to save values
* add save keys to store meta information for save values
* add save tuple to store save keys and values
* add save list to store lists of save tuples
* reimplement semi transparency to GUI elements (this does not fix the z-order problem)
Fix orientation and velocity debug rendering
Limit selection to best match (closest to camera)
Enable render context inheritance

* add custom render context
* bind debug rendering to key (F3 for now)
* fix velocity debug rendering