Make `IHasTooltip` interface use a `LocalisedString` instead of a regular string.

osu.Framework/Graphics/Cursor/IHasTooltip.cs

···
 // Copyright (c) ppy Pty Ltd <[e-mail]>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
+using osu.Framework.Localisation;
+
 namespace osu.Framework.Graphics.Cursor
 {
     /// <summary>
···
         /// <summary>
         /// Tooltip text that shows when hovering the drawable.
         /// </summary>
-        string TooltipText { get; }
+        LocalisableString TooltipText { get; }
     }
 }