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>

themes/m3ui-light/highlight.svg

@@ -0,0 +1,3 @@
+<svg width="32" height="32" viewBox="0 0 32 32" xmlns="http://www.w3.org/2000/svg">
+  <rect width="32" height="32" rx="10" fill="#7B294A" fill-opacity="0.84"/>
+</svg>