Load observersmappings from content types instead from hardcoded

This PR is proposed as a uncoupled alternative to the hard-coded-coupling PR https://github.com/joomla/joomla-cms/pull/1840 as a better fix for http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31603 .

It works as is, so could be ready for merging in a 3.3.x, but of course it would be better to also add the 2 columns "observers" of `#__extensions` and `#__content_types` to all database types for complete usability of it within e.g. in a 3.x.0 (x=4 or 5 e.g.).

It has big advantages to hard couplings, that are detailed in all good books about software patterns.