Add Material 3 candidate window addon for Fcitx5

Fork of the classicui addon (Fcitx5 5.1.23) with a Material 3 look, spring
based animations and a right click action menu for candidates.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/window.h

@@ -0,0 +1,33 @@
+/*
+ * SPDX-FileCopyrightText: 2016-2016 CSSlayer <wengxt@gmail.com>
+ *
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ *
+ */
+#ifndef _FCITX_UI_CLASSIC_WINDOW_H_
+#define _FCITX_UI_CLASSIC_WINDOW_H_
+
+#include <cairo.h>
+
+namespace fcitx::m3ui {
+
+class Window {
+public:
+    Window();
+    virtual ~Window() = default;
+
+    int width() const { return width_; }
+    int height() const { return height_; }
+    virtual void resize(unsigned int width, unsigned int height);
+
+    virtual cairo_surface_t *prerender() = 0;
+    virtual void render() = 0;
+
+protected:
+    unsigned int width_ = 100;
+    unsigned int height_ = 100;
+};
+
+} // namespace fcitx::m3ui
+
+#endif // _FCITX_UI_CLASSIC_WINDOW_H_