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.

manifest.json

@@ -3,7 +3,7 @@
   "name": "Scenario Studio",
   "version": "0.1.0",
   "author": "ct",
-  "description": "In-game scenario debugger and live editor: a non-modal dock over the running game with pause, step, breakpoints and restarts (with state rollback), the current scenario line, scene candidates with the reason they did or did not start, editable game state, an event log and the scene map. Edits go into folder mods or into a per-game studio layer over read-only mods, which can be exported as a mod. Author mode shows the script, novice mode the story moment.",
+  "description": "In-game scenario debugger and live editor: a non-modal dock over the running game with pause, step, breakpoints and restarts (with state rollback), the current scenario line, scene candidates with the reason they did or did not start, editable game state, an event log and the scene map. Edits go into folder mods or into a per-game studio layer over read-only mods, which can be exported as a mod; backgrounds dropped into a mod or the layer during the game are picked up with the \"Resources\" button of the transport bar. Author mode shows the script, novice mode the story moment.",
   "category": "tools, editor",
   "sources": [
     "src/**/*.cs"