shell: Initial control center implementation
shell: Integrate control center with shell
lookandfeel: Reindent greeter code
lookandfeel: Move more MessageBox logic into the component itself

This code was used by the login manager but it really belongs to
the component itself, hence it should be its default behavior and API.
lookandfeel: Use sizing from Plasma for MessageBox
lookandfeel: Refactor login manager layout and remove dialogs

Use a stack view to switch between the login screen and the
configuration screens.