Build fixes for CMake and GitHub Actions

[guihkx]

Sorry for the rather large pull request, but all the changes are kind of connected... So take your time to review it!

Possible highlights:

• CMake: Add an option to control which Qt version to use (-DUSE_QT_VERSION=5 or -DUSE_QT_VERSION=6)
• CMake/macOS: Add an option to control what macOS version to target (defaults to 10.15)
• CMake: Re-introduce the missing version string
• macOS/Windows: Re-introduce the missing app icon

Happy new year! 🎉

[zjeffer]

Shouldn't we also support ubuntu22.04?

[guihkx]

Shouldn't we also support ubuntu22.04?

You mean in the context of the CI pipeline? If so, I believe we already cover that:

• Qt 5: https://github.com/nuttyartist/notes/actions/runs/3822031085/jobs/6501762670
• Qt 6: https://github.com/nuttyartist/notes/actions/runs/3822031085/jobs/6501762733

Although I just noticed that the Qt 5 job is using ubuntu-latest instead of ubuntu-22.04, for some reason... lol

Currently it doesn't matter, since they refer to the same runner, but I think we should use ubuntu-22.04 on both, to avoid ambiguity.

[zjeffer]

Although I just noticed that the Qt 5 job is using ubuntu-latest instead of ubuntu-22.04, for some reason... lol

Currently it doesn't matter, since they refer to the same runner, but I think we should use ubuntu-22.04 on both, to avoid ambiguity.

Ah, I must have missed it.

[nuttyartist]

Works well on my Mac (: 👍 And happy new year!

[guihkx]

Awesome!

Thank you guys, I'll merge it then.