Improve build and packaging

• Updates lib2geom to latest commit (with version number)

• bumps lib2geom requirement so that it does not take (old) system lib and fail

• update the build process (on debian-based systems) to generate independent packages for inkscape, lib2geom, lib2geom-devel, inkscape-extensions, inscape-translations, themes, tutorials, examples, extension_manager.

• Todo: setup explicit "dependencies" and "recommended" tags

• Todo: think about combining inkscape-themes, inkscape-tutorials, inkscape-examples into a single "inkscape-extra" (?)

• Check that it does not break other packaging stuff