[FileExplorer] StlThumbnailProvider (#15568)

* Adds StlThumbnailProvider

* Spell checker fixes

* Adds missing changes

* Attempts to fix alpha background issue

* Adds missing dependency references

* Upgrades .NET Core 3.1 to .NET 5

* Updates Helix Toolkit to fix .net5 compatibility

* Return null bitmap If STL model is empty

src/settings-ui/Settings.UI.Library/PowerPreviewProperties.cs

@@ -131,6 +131,23 @@ namespace Microsoft.PowerToys.Settings.UI.Library
             }
         }
 
+        private bool enableStlThumbnail = true;
+
+        [JsonPropertyName("stl-thumbnail-toggle-setting")]
+        [JsonConverter(typeof(BoolPropertyJsonConverter))]
+        public bool EnableStlThumbnail
+        {
+            get => enableStlThumbnail;
+            set
+            {
+                if (value != enableStlThumbnail)
+                {
+                    LogTelemetryEvent(value);
+                    enableStlThumbnail = value;
+                }
+            }
+        }
+
         public PowerPreviewProperties()
         {
         }