`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.
