yoda: warning cleanup sweep (dead code + clippy --fix) (squashed)

Squash of 4 yoda commits:
- 84437e21 yoda: libcosmic-yoda dead-code purge (14->0 warnings)
- 999db0a4 yoda: cosmic-theme cleanup (4->0 warnings) — workspace at 0 warnings total
- 4743bb8e yoda: cargo clippy --fix on libcosmic-yoda (115->33 warnings)
- 675f3b59 chore: reduce local stack warnings

cosmic-theme/src/model/theme.rs

@@ -955,6 +955,12 @@ impl ThemeBuilder {
     }
 
     #[allow(clippy::too_many_lines)]
+    // The component_hovered/pressed_overlay vars are seeded once near the
+    // top of this fn and then reassigned inside each container block
+    // (primary, secondary, …) before being read again. The initial seed
+    // is therefore overwritten before any read, which is what the
+    // unused_assignments lint flags below.
+    #[allow(unused_assignments)]
     /// build the theme
     pub fn build(self) -> Theme {
         let Self {