Add build pipeline for macOS

This MR will create packaging/macos and populate it with a set of scripts and configuration files that make up the build pipeline for macOS. It will also change .gitlab-ci.yml to integrate this new pipeline. It contributes to issue #84 (closed).

Although I'm feeling confident merging this with Inkscape master, my work is still far from being finished, both in regards to the inner workings of the build pipeline as well as the app it produces. I propose that I create issues for all the things that I'm aware of or that we need to discuss instead of writing down all the details here. But to give you an (incomplete) overview:

• preparations for alpha testing (signing is minimum requirement to not scare of potential testers with "unidentified developer" message)
• fix things currently broken in the app (locale, extensions)
• dmg creation
• bring plist up to date
• analyze compiler warnings/messages (what needs fixing, what can be ignored)
• make build pipeline completely self-contained (get rid of currently required manual first-time setup)
• acquire runners for macOS (my machine at home isn't always available, but I'm working on getting us a runner that'll be nearly 24/7)
• collecting and working on user/tester feedback
• reaching out to build system maintainer
• compare library versions with other platforms, requirements/recommendations

So consider this a first step, a foundation that we can build upon.