Clean up and corner case bug fixes

[GwendolenLynch]

@SahAssar this is probably of most interest to you.

A few things:

• $config is deliberately a private variable, as YAML loading depends on getConfig() being called
• $app is the same, don't set/use a $this->app in the loader … that is not what it is there for
• Avoid using $app['request'] where ever possible, it will be populated regardless of where in the request cycle you are … the request stack however only gets set at the beginning of said cycle
• Always develop extensions with E_ALL, developer notices, and Twig strict variables turned on, or you have :koala: PRs :laughing:

[SvanteRichter]

Damn! Thanks so much! That's quite the cleanup!