Ensure dialog clean-up and allow overriding z-index

- Dialogs should actively be destroyed when destroying summernote (now that they are placed in `body`)
- By allowing to override the z-index, you can ensure that the dialog is placed in top of another dialog (e.g. if editor is in a dialog itself, see #677)

(note: my `dialogs` branch is based on the latest `develop` from @HackerWins , hence the many commits)