Move app quit to apprt action

This changes quit signaling from a boolean return from core app `tick()`
to an apprt action. This simplifies the API and conceptually makes more
sense to me now.

This wasn't done just for that; this change was also needed so that
macOS can quit cleanly while fixing #4540 since we may no longer trigger
menu items. I wanted to split this out into a separate commit/PR because
it adds complexity making the diff harder to read.

src/apprt/gtk/App.zig

@@ -1623,7 +1623,9 @@ fn gtkActionQuit(
     ud: ?*anyopaque,
 ) callconv(.C) void {
     const self: *App = @ptrCast(@alignCast(ud orelse return));
-    self.core_app.setQuit();
+    self.core_app.performAction(self, .quit) catch |err| {
+        log.err("error quitting err={}", .{err});
+    };
 }
 
 /// Action sent by the window manager asking us to present a specific surface to