AppFramework StreamResponse

AppFramework StreamResponse

First stab at the StreamResponse, see #12988

The idea is to use an interface ICallbackResponse (I'm not 100% happy with the name yet, suggestions?) that allow the response to output things in its own way, for instance stream the file using readfile

Unittests are atm lacking, plan is to

check if a mock of ICallbackResponse will be used by calling its callback (also unhappy with this name) method
Usage is:

$response = new StreamResponse('path/to/file');

rename io to output, add additional methods and handle error and not modified cases when using StreamResponse

fix indention and uppercasing, also handle forbidden cases

fix indention

fix indention

no forbidden, figuring out if a file is really readable is too complicated to get to work across OSes and streams

remove useless import

remove useless import

fix intendation