meson: install macOS app bundle

[Atemu]

Expected behavior of the wanted feature

Currently, there is the osxbundle.py tool which packagers must "manually" execute in their packaging scripts.

As a packager, I want generation and and installation of the mpv.app bundle on macOS/Darwin to be integrated into the build system so that I have a standardised, clean and less error-prone way to distribute them to users.

Alternative behavior of the wanted feature

The bundle could be a separate install target rather than being integrated into the regular install target.

[Akemi]

i am not sure how our stance is atm, but we don't provide something like this for windows (completely self contained executable or installation process) and linux (Appimage etc) either. first and foremost mpv is a CLI player and is treated like this to this day.

the only benefit i see integrating the bundeling into the build system, is it being called via waf or meson itself. though it won't be "cleaner" or "less error-prone" than a separate called script.

like i mentioned in the closed issue, i believe the whole bundling script should be rewritten. atm it's very much some kind of frankenstein monster that grew over time. besides the swift parts, that were added a lot later, everything could probably be done in a lot nicer way.

[Akemi]

just a heads up, i added a custom target in my PR #13529

[Atemu]

Thank you!