Add Mac build script for CI and development

[cafca]

Adds scripts/build_macos.sh, which builds Reflection for macOS using Meson. Adds a CI step to build for mac and upload the resulting app bundle to Github for new commits on main.

Adds a workflow_dispatch trigger to the CI task so that it can be triggered manually on any branch from Github's web UI.

Uses the system keyring on macOS for storing user credentials. This means that a popup asks the user if they want to allow Reflection access to its keyring when the app starts.

Updates resource loading to work in macOS app bundles where the app may be moved around to any location in the system and paths need to be relative to the bundle for that to work.

Caveats

• The resulting Mac app is not notarized and I had to explicitly allow it on the commandline with sudo xattr -rd com.apple.quarantine /Applications/Aardvark.app unless I was using a build made on the same machine
• No use of macOS system preferences for datetime formatting or fonts, this just uses the fallbacks.

Todo

Still needs some cleanup, documentation, and rebasing to remove unrelated changes.

[jsparber]

I know the MR it's still a draft. Reflection will stick to GNOME's commit message style. Shouldn't be too much work since the commit history still needs cleanup anyway.