`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.
As long as this isn't a constructor parameter it feels best to
gracefully handle omission. Realistically having it in the ctor is the
best move, but it doesn't feel great in line with the other parameters
passed in via object initalisers.
