Unify mixed color selection

Use the shared color selector for mixed property values and remove the legacy inline palette and its state.

src/app/mod.rs

@@ -2523,8 +2523,6 @@ pub enum Message {
     /// properties-panel dropdown (a combo_box) opens, so at most one panel
     /// dropdown is open at a time and they can't overlap. (#235)
     PropColorPickerClose,
-    /// Toggle the full ACI colour palette expansion.
-    PropColorPaletteToggle,
     /// Enter the model-space editing mode inside the given viewport (MSPACE).
     EnterViewport(acadrust::Handle),
     /// Exit MSPACE and return to paper-space editing (PSPACE).