Incompatible change in JarPackager API

Hi,
I'd like to make the following incompatible change in JarPackager APIs 
motivated by a real api misuse.

- change ProjectContent from a subclass of AbstractCompositeContent to a 
subclass of plain JarContent
- the particles of ProjectContent won't be generic JarContents but 
ProjectContent.Particles.

This will
- enable compile time checking of the filter and mapper used with 
ProjectContent.Particle instead of a runtime check in the current 
implementation
- make the apis more readable by making it obvious what classes to use 
merely by signatures of methods instead of comments in the documentation 
, which isn't of course read by anybody.

As a side effect, I will also remove the class AbstractCompositeContent. 
As ProjectContent is not an AbstractCC anymore, ACC has just one 
subclass, CompositeContent. Thus, ACC and CC will be merged into final 
class CompositeContent with no possibility of extension.

All impacted modules on the branch (web) will be corrected appropriatelly.

If you have any objections against this change, please speak up now or 
forever hold your peace ;-)

- Ondra