Improve Import options

- Added Import options window
- Changed cache logic:
  - single cache file instead of separate cache files for each bundle
  - program folder as a default cache folder
  - added an option to specify a custom cache folder

AssetStudioGUI/Properties/Settings.Designer.cs

@@ -370,5 +370,17 @@ namespace AssetStudioGUI.Properties {
                 this["rawByteArrayFromMono"] = value;
             }
         }
+        
+        [global::System.Configuration.UserScopedSettingAttribute()]
+        [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]
+        [global::System.Configuration.DefaultSettingValueAttribute("")]
+        public string cachePath {
+            get {
+                return ((string)(this["cachePath"]));
+            }
+            set {
+                this["cachePath"] = value;
+            }
+        }
     }
 }