macOS: new window can inherit font size

This is the second part of #281 and adds the same mechanism that was
added for tabs in #296 for windows.

It works exactly the same way.

src/input/Binding.zig

@@ -277,6 +277,7 @@ pub const Key = enum(c_int) {
     copy_to_clipboard,
     paste_from_clipboard,
     new_tab,
+    new_window,
 };
 
 /// Trigger is the associated key state that can trigger an action.