filters: add selected and length filters

We will be using them in a followup commit.
delete-modal: let user confirm deletion (fixes #84)
container: make delete button a default botton

This button is no longer dangerous in itself since we now show a
confirmation dialog.
swift: add deleteDirectory method

This deletes all objects in the specified pseudo-directory.
delete-modal: support deleting pseudo-directories (fixes #36)

This is not terribly elegant at the moment: you simply see the
directory as an item in the delete modal and when it is deleted, it
disappears from the object listing.

See #85 for ideas to improve this.
Merge pull request #86 from mgeisler/delete-modal

Delete modal