Add dragging window with cursor feature (#1840)

* X11 implementation.

* Introduce example.

* Wayland implementation.

* Windows implementation.

* Improve Wayland seat passing.

* MacOS implementation.

* Correct windows implementation per specification.

* Update dependency smithay-client-toolkit from branch to master.

* Fixed blocking thread in windows implementation.

* Add multi-window example.

* Move Wayland to a different PR.

* Fix CHANGELOG.

* Improve example.

Co-authored-by: Markus Røyset <maroider@protonmail.com>

* Rename `set_drag_window` to `begin_drag`.

* Improve example.

* Fix CHANGELOG.

* Fix CHANGELOG.

Co-authored-by: Markus Røyset <maroider@protonmail.com>

* Rename to `drag_window`.

* Fix typo.

* Re-introduce Wayland implementation.

* Fixing Wayland build.

* Fixing Wayland build.

* Move SCTK to 0.12.3.

Co-authored-by: Markus Røyset <maroider@protonmail.com>

src/platform_impl/web/window.rs

@@ -222,6 +222,11 @@ impl Window {
         }
     }
 
+    #[inline]
+    pub fn drag_window(&self) -> Result<(), ExternalError> {
+        Err(ExternalError::NotSupported(NotSupportedError::new()))
+    }
+
     #[inline]
     pub fn set_minimized(&self, _minimized: bool) {
         // Intentionally a no-op, as canvases cannot be 'minimized'