Use Declaration Merging Instead of Generics

[FlorianRappl]

Description

The current API uses generics, which makes it hard to deal with it on multiple layers. Especially from a pilet POV its a huge pain.

We need a better way - that originates the API already from piral-core and only extends it all the way.

Background

Right now the app shell should / would alias the "final" API and use it in generics. As such MyAppShellApi becomes a thing. It should not be. It will prevent cross Piral solutions (pilets) independent of any pattern libraries or other dependencies - just because the APIs are named differently (but share the same core or are even 100% the same).

Discussion

This will have an impact on how pilet API extensions are written. The intention here is not to make things more complicated or worse, but to simplify.

By using declaration merging we are on the road to have a single Piral instance only (which makes sense - multiple instances are academic anyway).

[FlorianRappl]

Landed in develop.