Create custom cursor with directly with event loop

Replace the `CustomCursorBuilder` with the `CustomCursorSource` and
perform the loading of the cursor via the
`EventLoop::create_custom_cursor` instead of passing it to the builder
itself.

This follows the `EventLoop::create_window` API.

tests/send_objects.rs

@@ -31,6 +31,6 @@ fn ids_send() {
 
 #[test]
 fn custom_cursor_send() {
-    needs_send::<winit::window::CustomCursorBuilder>();
+    needs_send::<winit::window::CustomCursorSource>();
     needs_send::<winit::window::CustomCursor>();
 }