Spell Activation Overlays (SAOs) could be selected individually.

These options would be set in the Interface > AddOns > SpellActivationOverlay panel, with a new subcategory for each class.

This issue is the SAO counterpart of the GAB evolution described in #61.

Implemented in #90.
Each class only sees its own Spell Alerts.
They are setup in the top-right corner of the options panel.
Spell Alerts are grayed out if the Spell Alert Opacity slider is set to "Off". This behavior is identical to when the "Toggle Test" button is grayed out.