Reboot LibreOffice packaging

LibreOffice upstream is now almost fully migrated to gbuild. This would allow us to do real partial builds of LibreOffice, with split source packages. There are advantages and disadvantages to such an approach.
Disadvantages:
- More source packages mean more package handling as with an upstream release more source packages need to be build and tested
- testing: most tests require a full libreoffice installation, thus a partial source package cant be tested completely during its build, thus these packages need to be staged in -proposed.
Advantages:
- Bugfixes would be possible without full rebuilds
- Updates can be done without pushing a all libreoffice packages again to all users
- Faster build times as some parts of libreoffice can be build on multiple builds in parallel
- Filesystem limitations on e.g. PPA builders would not hurt us that hard anymore (as the biggest part of the fs-use is objects and dependencies)

And initial proposal would be to have the following eight source packages:
- libreoffice-dev-internal: libreoffice headers
- libreoffice-core: ure and core libraries
- libreoffice-writer, libreoffice-calc, libreoffice-base, libreoffice-impress: applications (impress includes draw, writer includes math) -- these could then build in parallel
- libreoffice-l10n: localization
- libreoffice-common: all the misc. bits and pieces.

And yes, this would require a rewrite of the LibreOffice packaging from scratch -- but that might also simplify things a lot.