New system "darts.lib.mixin-traits"

The new system contains a new implementation for "annotatable",
and the implementation of "documentable" has been moved there
as well. Also, there is a new trait "observable".

The old implementation of "annotatable" should be considered
deprecated. It may be kept around for a little while, until I
have modified all the code using it. For most of the code, it
should be as simple as doing a shadowing import of the symbols
from darts.lib.mixin-traits. A few corner cases may exist,
though.