feat(window): add is_focused API (#407)

plugins/window/src/desktop_commands.rs

@@ -112,6 +112,7 @@ getter!(outer_size, PhysicalSize<u32>);
 getter!(is_fullscreen, bool);
 getter!(is_minimized, bool);
 getter!(is_maximized, bool);
+getter!(is_focused, bool);
 getter!(is_decorated, bool);
 getter!(is_resizable, bool);
 getter!(is_visible, bool);

plugins/window/src/lib.rs

@@ -40,6 +40,7 @@ pub fn init<R: Runtime>() -> TauriPlugin<R> {
                         desktop_commands::is_fullscreen,
                         desktop_commands::is_minimized,
                         desktop_commands::is_maximized,
+                        desktop_commands::is_focused,
                         desktop_commands::is_decorated,
                         desktop_commands::is_resizable,
                         desktop_commands::is_visible,