Refactor Core API

I've made a list of potential changes which need to be done to cleanup the overall API and make it more consistent. For example all handles should be used via functions in the Core object which forwards to all the managers (they should all be accessible via Core but not a part of the public API!).

Template abstractions for complex handles like Buffers or Images will also use the functions in the Core object instead of working with managers directly. They will become shallow interfaces managing type safety - that's it. They will also get copy- and move-constructors as well as -operators. This will be possible via type verification in the debug build (https://stackoverflow.com/questions/1055452/c-get-name-of-type-in-template).

First changes might be made to allow more asynchronous execution of some functions as planned in #43.