Add a "quick add" option. At this time the option is available on list

group headers. The quick add dialog provides a title field and a list
selector. There are buttons to save the new task and continue with
another one and to open the full blown editor.

In future we should add a way to provision the quick add dialog with
more values and allow to disable or hide the spinner. That way we can
use it to add sub tasks quickly or to add it to other group section
headers as well.