Refactoring; structs for popups/multi-selects, additional popups in places

src/events.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "document_manager.h"
+#include "imgui.h"
 #include "resources.h"
 #include "settings.h"
 
@@ -8,7 +9,8 @@ namespace anm2ed::events
 {
   class Events
   {
-    ImGuiSelectionExternalStorage storage{};
+    imgui::MultiSelectStorage storage{};
+    std::set<int> unusedEventIDs{};
 
   public:
     void update(document_manager::DocumentManager& manager, settings::Settings& settings,