swift: add createContainer method
RootCtrl: indent like other controllers
root: allow creating containers (fixes #122)
create-container: add form validation logic
create-container: submit modal when pressing the Enter key

This required making the buttons part of the form: pressing enter in
an input field will trigger the click handler for the first button
inside the form. Changing the structure like this does not affect the
layout since the form element itself doesn't have any styling.
Merge pull request #130 from mgeisler/create-container

Allow creating Swift containers