winit-core:winit: add tablet input support

The API is integrated into the `WindowEvent::Pointer*` API and is
present in form of `TabletTool` variant on corresponding data entries.

For now implemented for Web, Windows, and with limitations for Wayland.

Fixes #99.

Co-authored-by: daxpedda <daxpedda@gmail.com>

winit-web/src/cursor.rs

@@ -216,7 +216,7 @@ impl CursorHandler {
             Cursor::Custom(cursor) => {
                 let cursor = match cursor.cast_ref::<CustomCursor>() {
                     Some(cursor) => cursor,
-                    None => todo!(),
+                    None => return,
                 };
 
                 if let SelectedCursor::Loading { cursor: old_cursor, .. }