Mostly as discussed in the dev chat. Allows users to run custom startup logic.

Also orders AppBuilder.__slots__ alphabetically

Checklist for PRs

• Added .. versionadded:: version, .. versionchanged:: version or .. deprecated:: version to the docstrings for user facing changes (for methods/class descriptions, arguments and attributes) Only changed classes that already have the "new in v20" directive"
• Created new or adapted existing unit tests
• Documented code changes according to the CSI standard
• Added myself alphabetically to AUTHORS.rst (optional)
• Added new classes & modules to the docs and all suitable __all__ s