Support file-less scenes. Add scene split, merge and reassign file (#3006)

* Reassign scene file functionality
* Implement scene create
* Add scene create UI
* Add sceneMerge backend support
* Add merge scene to UI
* Populate split create with scene details
* Add merge button to duplicate checker
* Handle file-less scenes in marker preview generate
* Make unique file name for file-less scene exports
* Add o-counter to scene update input
* Hide rescan for file-less scenes
* Generate heatmap if no speed set on file
* Fix count in scene/image queries

ui/v2.5/src/components/MainNavbar.tsx

@@ -95,6 +95,7 @@ const allMenuItems: IMenuItem[] = [
     href: "/scenes",
     icon: faPlayCircle,
     hotkey: "g s",
+    userCreatable: true,
   },
   {
     name: "images",