FIX: Show RGB for custom color and allow staying in official picker dialog

- Display RGB values when a custom color is selected
- Prevent official color picker dialog from closing when custom color dialog is canceled
- Ensure multi-color materials are correctly displayed when importing new 3MF files

jira: STUDIO-12938, STUDIO-12937, STUDIO-12933, STUDIO-13110

Change-Id: Iab410585bf8cc5e9e81c6f0da23fe4ddba561785
(cherry picked from commit 2c20d591b998b02b0b66ac81b048a28fa26bd409)

src/slic3r/GUI/FilamentPickerDialog.hpp

@@ -56,6 +56,7 @@ private:
 
     // UI update methods
     void UpdatePreview(const FilamentColorCode& filament);
+    void UpdateCustomColorPreview(const wxColour& custom_color);
     void UpdateButtonStates(wxBitmapButton* selected_btn);
 
     // Shaped window methods
@@ -64,6 +65,7 @@ private:
 
     // Data loading
     bool LoadFilamentData(const wxString& fila_id);
+    wxColourData GetSingleColorData();
 
     // UI elements
     wxStaticBitmap* m_color_demo{nullptr};