Draft: Asset handle

Requires: !93 (merged)

This MR adds a way for creating static-handle classes (which could be used to hide internal implementation, like Handle class does, but this time for static classes). It is a mix of a bridge and a facade pattern. It gives the opportunity to hide complex code related to Asset and Token classes.

If you are checking these changes by commits, start from the one named Move asset.py to it's own package, cause this MR should be rebased when !93 (merged) will be delivered.