Studio: backgrounds dropped during the game are picked up by the Resources button

Layer gets backgrounds/; StudioBackgrounds re-reads the file list and the background
cache and shows the current background (with _depth/_albedo) anew. The Resources
button sits on the transport bar (every tab); a new scene picks its background from
a list with the source mod. studio_live covers drop, reload, replace and export.

src/Model/StudioAssetReload.cs

@@ -0,0 +1,15 @@
+namespace Cthangover.Mods.ScenarioStudio
+{
+    using System.Collections.Generic;
+
+    /// <summary>
+    /// What the "Resources" button found: how many backgrounds the game has now and
+    /// which of them appeared since the last read (files dropped in while the
+    /// game ran). A replaced picture is not "added" — it is simply shown anew.
+    /// </summary>
+    public sealed class StudioAssetReload
+    {
+        public int Total { get; set; }
+        public List<string> Added { get; set; } = new();
+    }
+}