Refactored and made the delegator a bit easier

This MR introduces a small refactor to the current rather fragile backend selection (we had to distinguish to many special cases, so no its a bit more stream lined) I eradicated the invalid states in the respective enums, to make the code a bit less error prone.

Main Changes:

• We now have a backend selection enum thats decoupled from the application nesting (very important as they are actually decoupled)
• we always keep a stack instead of only for properly nested regions.
• modified the test to test a mixed mode