swift: add deleteContainer method

The method will first empty the container before deleting it.

Should the container be modified in the middle of this operation (by
adding a new object while it is being emptied), the operation will
fail. This is not yet tested.
swift-sim: add support for deleteContainer
root: add delete container modal (fixes #37)

For now, this works very similarly to the delete object modal. It
should be extended with a progress bar later.
Merge pull request #146 from mgeisler/delete-container

Add support for deleting containers