Small refactoring of the Find dialog:
- implement an own sorting+proxy strategy for the list using the Qt4 interview framework (document how to make the filter by type working with the new system, it was not working even before)
- delay the on-type search with a 0.5s timer, so it's a bit more usable
- disable the Ok button if no astro body is selected (eg when the list has no search result)

I hope it's all OK...