feat: Add drag/drop functionality to the screenshot viewer (#536)

* feat: Add drag/drop functionality to the screenshot viewer

* feat: Add arrow key controls for screenshot carousel

CefCustomDragHandler.cs

@@ -0,0 +1,43 @@
+using CefSharp.Enums;
+using CefSharp;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace VRCX
+{
+    /// <summary>
+    /// This class is used to 'handle' drag and drop events.
+    /// All it does is call a function in the app with the file name of the file being dragged into the window, since chromium doesn't have access to the full file path on regular drop events.
+    /// </summary>
+    public class CustomDragHandler : IDragHandler
+    {
+        public bool OnDragEnter(IWebBrowser chromiumWebBrowser, IBrowser browser, IDragData dragData, DragOperationsMask mask)
+        {
+            if (dragData.IsFile && dragData.FileNames != null && dragData.FileNames.Count > 0)
+            {
+                string file = dragData.FileNames[0];
+                if (!file.EndsWith(".png") && !file.EndsWith(".jpg") && !file.EndsWith(".jpeg"))
+                {
+                    dragData.Dispose();
+                    return true;
+                }
+
+                // forgive me father for i have sinned once again
+                AppApi.Instance.ExecuteAppFunction("dragEnterCef", dragData.FileNames[0]);
+                dragData.Dispose();
+                return false;
+            }
+
+            dragData.Dispose();
+            return true;
+        }
+
+        public void OnDraggableRegionsChanged(IWebBrowser chromiumWebBrowser, IBrowser browser, IFrame frame, IList<DraggableRegion> regions)
+        {
+
+        }
+    }
+}