MacOS: headless mode UX improvements

[ogapo]

Don't show the application in the dock or task switcher unless one of the following is true:

• we create a non-headless display driver or
• an alert must be shown

This is generally best practice, but also greatly improves the UX of godotTools.lsp.headless mode. Without this fix, enabling headless mode for the GodotTools language server (which is quite nice) causes a phantom Godot application (no icon) to appear to be running in the dock / task switcher whenever VSCode is open.

git history shows this call used to be in display_server_macos and was moved to os_macos when alert was moved to OS. Because if this I think it should be a fairly safe change.

[fire]

Test plan:

1. Go to Mac.
2. Build this Godot engine pull request.
3. Launch the Godot Engine headless.
4. See if any windows or icons show.

[ogapo]

Godot should be always bundled to function properly

Just an aside, but I think it would be good to not make the assumption that users are always running bundled (in this case it's fine since I think the default mode for unbundles is akin to accessory). Anyone running a custom godot build for their team (myself included) may prefer not to bundle since compiling godot locally is part of our Debug/Run flow.