display order for packages

This pull request isn't a very complete thing, so far it only adds a column which you can only edit using some
SQL hacking. I just wanted to get the discussion started - I'd be happy to extend this pull request as we go..

Why do I need this? Right now concrete/startup/packages.php fetches all available packages and calls the on_start method if available.

I've just had a situation where I have to do some kind of magic logging, no details but believe me, it has to be in on_start for a number of reasons. A couple of things rely on multilingual, constants set by another package. My own package doesn't work without them.

I thought about adding something like package dependencies, that would be awesome but a lot more complicated. Having a field in the Packages table is a quick and dirty solution, but as I said, I'd be happy to improve this..