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-win32/src/window.rs

@@ -1263,6 +1263,7 @@ impl InitData<'_> {
             _file_drop_handler: file_drop_handler,
             userdata_removed: Cell::new(false),
             recurse_depth: Cell::new(0),
+            last_tablet_down_button_state: Cell::new(0),
         }
     }