Remove implicit operator from bindables

[UselessToucan]

resolves #2150

In order to avoid accidental misuse / misunderstandings, .Value must always be added to Bindable when requesting its value. One such case which used to be confusing:

Bindable<Drawable> bindableDrawable = new Bindable<Drawable>();

if (bindableDrawable != null)
{
    // true
}

if (bindableDrawable.Value != null)
{
    // false
}

Drawable drawable = null;

if (bindableDrawable == drawable)
{
    // ???
}