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>

NOTICE.md

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+# Notice
+
+`src/` is a modified copy of the `classicui` addon of
+[Fcitx5](https://github.com/fcitx/fcitx5) 5.1.23
+(`src/ui/classic`), copyright the Fcitx5 authors including CSSlayer, licensed
+under LGPL-2.1-or-later. Files keep their original SPDX headers. The changes are
+the renamed addon, the Material 3 painting, the animations and the candidate
+action menu.
+
+The themes in `themes/` are derived from the layout values of a local theme and
+use the same license.
+
+`kwin/install-fadingpopups-override.sh` patches a copy of KWin's *Fading Popups*
+effect (GPL-2.0-or-later, KDE) on the user's machine. No KWin code is included
+in this repository.