[backend] implement product description parsing for source server
[api] basic product tracking
[api] find patchinfo packages by package kind and not via package name convention
[api] basic channel tracking
[api] add "addchannels" api call, which does copy matching channel packages and repositories
[api] avoid submit actions which do not change the source.

Add special support for submitting changes of channel packages as submit action
[api] take care about the channel release target definition
[api] filter channel packages based on maintained projects list
[backend] products define product dependencies per entire product in future, not per media
[api] code cleanup
[api] avoid multiple object lookups for commands