Use macOS cooperative app activation to activate app to foreground

[ychin]

macOS 14 Sonoma deprecated the NSApplication API activateIgnoringOtherApps, and provides a new activate API along with a cooperative yieldActivation command that can be called by the host app. We should switch to using this API.

One issue is that calling activate requires the foreground app to cooperate and willingly yield. There are a few usages of activateIgnoringOtherApps right now:

1. Using the "New Window" menu item on the Dock icon. I think this actually works automatically and the activate call may be superfluous.
2. Opening a new Vim from the mvim script so the new Vim window would be in foreground. This wouldn't work as the terminal has foreground.
3. Invoking :call foreground() in Vim. This also wouldn't just work as Vim does not have the right to gain activation when it's in background under this API model.
4. When opening a file by dragging it to the dock, etc. This should work as MacVim should gain focus when that happens.
5. Running the new XCTest test cases. This also wouldn't just work if running the tests from terminal (e.g. in CI we just run make -C src macvim-tests) as MacVim will fail to gain foreground this way. Running from Xcode will be fine though as Xcode will yield.

To fix 2/3/5, we probably need to run a simple Swift script that calls NSApplication.shared.yieldActivation(toApplicationWithBundleIdentifier: "org.vim.MacVim") to manually force the terminal to yield to MacVim. For 2 and 5, we can call them as part of the mvim script and Makefile, respectively.

For 3, it's a little tricky. If the foreground request is invoked as a remote call using the --remote family of commands, it would work, but MacVim will lose the ability to randomly bring itself to the foreground at will (e.g. running :sleep 2 | call foreground() would not work). This is probably ok as the foreground() API doesn't work on Win32 too and in general it's best to let the foreground app has agency over when focus unless the user manually clicked away.

Currently, in macOS 14, the activateIgnoringOtherApps API is only deprecated but it still works so there's no reason to do anything. Just keep this in mind for now. If macOS 15 formally breaks it, we will implement the fix along with the manual yielding from terminal so that calling mvim from terminal, running tests etc would still work. We don't really want to do anything until this API breaks because even fixing it "properly" we will still lose case (3) where call foreground() will stop working if not called using --remote.