Fix session feature

[fracarnicelli]

In "Runner.py" there's the need to improve the execution flow of "main_execution" method. Once the the user prints the edited image, the script ends without giving the opportunity to start another session.

[GassiGiuseppe]

I think that what we are reffering here is the fact that we implemented a session id for managing multiples session, this was done beacuse we where creating a way to manage multishots and each multishot correspond to a session, such implementation is not fully created as our first tought. In this panorama the function to increment the session id is already written: https://github.com/gdgbari/photobooth/blob/master/FolderManager.py#L83 here but currently there isnt a place where this is called

[GassiGiuseppe]

Moved this into future enhancment since it belongs to the feature of different sessions

[vitodibari]

Rethink the session management process