Add "import all skins from stable" option (and mass delete)

osu.Game/Database/ArchiveModelManager.cs

@@ -5,6 +5,7 @@ using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
+using System.Threading.Tasks;
 using JetBrains.Annotations;
 using Microsoft.EntityFrameworkCore;
 using osu.Framework.IO.File;
@@ -283,7 +284,7 @@ namespace osu.Game.Database
             var notification = new ProgressNotification
             {
                 Progress = 0,
-                CompletionText = "Deleted all beatmaps!",
+                CompletionText = $"Deleted all {typeof(TModel)}s!",
                 State = ProgressNotificationState.Active,
             };
 
@@ -385,6 +386,41 @@ namespace osu.Game.Database
             return fileInfos;
         }
 
+        #region osu-stable import
+
+        /// <summary>
+        /// Set a storage with access to an osu-stable install for import purposes.
+        /// </summary>
+        public Func<Storage> GetStableStorage { private get; set; }
+
+        /// <summary>
+        /// Denotes whether an osu-stable installation is present to perform automated imports from.
+        /// </summary>
+        public bool StableInstallationAvailable => GetStableStorage?.Invoke() != null;
+
+        /// <summary>
+        /// The relative path from osu-stable's data directory to import items from.
+        /// </summary>
+        protected virtual string ImportFromStablePath => null;
+
+        /// <summary>
+        /// This is a temporary method and will likely be replaced by a full-fledged (and more correctly placed) migration process in the future.
+        /// </summary>
+        public Task ImportFromStableAsync()
+        {
+            var stable = GetStableStorage?.Invoke();
+
+            if (stable == null)
+            {
+                Logger.Log("No osu!stable installation available!", LoggingTarget.Information, LogLevel.Error);
+                return Task.CompletedTask;
+            }
+
+            return Task.Factory.StartNew(() => Import(stable.GetDirectories(ImportFromStablePath).Select(f => stable.GetFullPath(f)).ToArray()), TaskCreationOptions.LongRunning);
+        }
+
+        #endregion
+
         /// <summary>
         /// Create a barebones model from the provided archive.
         /// Actual expensive population should be done in <see cref="Populate"/>; this should just prepare for duplicate checking.