`opacity` issues on disabled controls

[Malvoz]

UAs use opacity on disabled controls, but it doesn't work well on the map controls for the following reasons:

1. Unlike UAs we use aria-disabled and not disabled, so the controls remain focusable, causing a barely visible outline on focus (because the outline too has the opacity applied).

2. Too low contrast on the Reload button (because it has both a lighter color + opacity as of Refactoring HTML anatomy of items in layers #531).

3. Creates a stacking context (I believe) on disabled controls in the layer control, causing underlying layer setting buttons to show through the overlaying layer item on drag.