Package handling

I think we should polish the way of handling packages in appNG.

Currently packages are accessible only in their corresponding repository. An aggregated view of all packages does not exist.

This has some disadvantages, f.e.:

• Find a package/-version:
  To determine the latest package version we have to browse every available repository and search for the corresponding package.
• Performance:
  Everytime we browse a repository (especially remote) the package list must be generated. As far as I know we have some kind of cache or polling mechanism in the appng manager application but I guess this is not the case when using appNGizer?

One solution would be to implement a new entity /package where all packages of all repositories are aggregated and available.

The same behaviour would be if we go further down to a package /package/<package_name>.

As this involves changes in several appng components (core, template, manager, ...) I think we should break down this issue in several subissues and use this only as base for generall discussion and brainstorming of this topic.