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.

src/platform_impl/orbital/event_loop.rs

@@ -22,7 +22,7 @@ use crate::{
         Key, KeyCode, KeyLocation, ModifiersKeys, ModifiersState, NamedKey, NativeKey,
         NativeKeyCode, PhysicalKey,
     },
-    window::WindowId as RootWindowId,
+    window::{CustomCursor as RootCustomCursor, CustomCursorSource, WindowId as RootWindowId},
 };
 
 use super::{
@@ -805,6 +805,13 @@ pub struct ActiveEventLoop {
 }
 
 impl ActiveEventLoop {
+    pub fn create_custom_cursor(&self, source: CustomCursorSource) -> RootCustomCursor {
+        let _ = source.inner;
+        RootCustomCursor {
+            inner: super::PlatformCustomCursor,
+        }
+    }
+
     pub fn primary_monitor(&self) -> Option<MonitorHandle> {
         Some(MonitorHandle)
     }