WIP: Package Fractal for mac in an .app bundle

There are multiple things in this MR that could be split into multiple ones. For example, I wanted to create the Info.plist file with the metadata we have in meson, and therefore prepared for that in 4edd1220 by doing the same thing for the .appdata.xml (@danigm you might want to look at this)

I don't have time right now to list what is working or not nor to clean up the MR a bit, but I listed what's done and what's not earlier today in #233

For the script that gathers all the dependencies, I was heavily inspired by what the CoyIM project does

How to build this:

You'll need homebrew with a bunch of dependencies (brew install gtk+3 gnome-icon-theme).

./scripts/build-mac.sh /tmp/Fractal.app _mac_build
ninja -C _mac_build install
open /tmp/Fractal.app

Keep in mind that we still need to create a wrapper that sets a bunch of environment variables and set up code singing in order to make the .app work on other machines that don't have GTK installed through Homebrew.