lot stuff (recurrence, pkg version check, introtext, overridables)

- recurrence icon and handling (issue #627)
- NEW content plugin (to handle deletion of events when part of recurrence) (issue #627)
- add script to package for version check (to stop mod/plg installation when component would stop) (issue #632)
- don't show foreign introtext (issue #628)
- allow overriding of common layouts too (e.g. list of events) (issue #630)