[macOS] Align menu items between separators.

[bruvzg]

Mostly relevant in combination with #65229

Before	After

Built-in Godot menus align everything to an icon, but aligning only items between separators matches the way default macOS app handle it:

[AdriaandeJongh]

Reviewed based on the screenshots. I'm not 100% sure about how the Export as > doesn't get indented when the rest of the menu does, but regardless: this helps making the more important options more prominent.

Didn't look at the code.

[bruvzg]

I'm not 100% sure about how the Export as > doesn't get indented when the rest of the menu does, but regardless: this helps making the more important options more prominent.

That's how it is done in most macOS apps, but aligning everything might be better (and can simplify code a bit).