Improve channel manager so that is no longer possible to publish invalid components (e.g. components with empty required fields)

When a component with required fields is added from the channel manager, it is possible to leave these required fields empty (for example by clicking on close or by navigating away from the channel manager). The initial state, with empty or invalid fields, are stored in the channel manager. When the changes in the channel manager get published, these invalid components also get published.

On publication there is currently no check on invalid components. Therefore it is possible that these invalid components get published, although they don't meet the requirements.