FIX: modify UI of FilamentMapDialog

1. Modify dragging behavior
2. The swap button is grayed out in automatic mode
jira:none

Change-Id: Idff795bf092c4959c67d0711b0be316fba997684
(cherry picked from commit c7486e1c5e56667cab61ee9a97898f03b65b64b8)

src/slic3r/GUI/FilamentMapDialog.hpp

@@ -45,6 +45,7 @@ private:
     DragDropPanel* m_auto_left_panel;
     DragDropPanel* m_auto_right_panel;
     Button       * m_switch_filament_btn;
+    Button       * m_switch_filament_btn_auto; // for placeholder
 
 private:
     const DynamicPrintConfig* m_config;