[backend] fix full tree rebuild handling of meta files
refactor sync_to_stage

- also allow multiple stage servers and stage server partitioning
- also support a sync script
[backend] implement appdata-icon extraction

Unextracted icons in the appdata file make the xml invalid.

This commit also changes the appdata parsing from XML::Structured
to XML::Simple, as the appdata specification is rather complex and
in flux, so we do not want to have its dtd in the build service.
require perl-XML-Simple for the publisher
[dist] fix missing new build requires for publisher
[backend] simplify supportstatus handling: transport it with the updateinfo
[backend] fix supportstatus stripping code