Do not store direct references to original bindable

`DifficultyAdjustSettingsControl` and its inner `SliderControl` were
holding different references to `DifficultyBindable`s from the
difficulty adjust mod, therefore leading to bindings being lost to the
framework-side automatic unbind logic if the mod was toggled off and
back on in rapid succession.

Resolve by adding a shadowed implementation of `GetBoundCopy()` and
using it to isolate the controls from the mod bindable.

osu.Game/Rulesets/Mods/DifficultyAdjustSettingsControl.cs

@@ -41,10 +41,10 @@ namespace osu.Game.Rulesets.Mods
             {
                 // Intercept and extract the internal number bindable from DifficultyBindable.
                 // This will provide bounds and precision specifications for the slider bar.
-                difficultyBindable = (DifficultyBindable)value;
+                difficultyBindable = ((DifficultyBindable)value).GetBoundCopy();
                 sliderDisplayCurrent.BindTo(difficultyBindable.CurrentNumber);
 
-                base.Current = value;
+                base.Current = difficultyBindable;
             }
         }