By default, ToggleGroup is coded to activate/deactivate the container for all associated Toggle elements. When the container is deactivated, all contained Toggles will get their OnDisable called. This will cause the Toggle to unregister itself from the ToggleGroup.

When a Toggle calls UnregisterToggle() on it's ToggleGroup, it is removed from the ToggleGroup's internal list.

Previously, any time UnregisterToggle() was called on a ToggleGroup that is set to disallow an off state, the group will then scan its list for at least one checked Toggle. Because the checked toggle is ultimately removed from the list before the list is totally emptied, the group will attempt to fix itself and set the first Toggle in the list to be On. This creates unpredictable behavior, both because the order of the list is non-deterministic, and because the first list element is rarely the value that should be set to On.

This PR moves the logic for ensuring that one Toggle is marked On to a new function. The only times that this function actually needs to be invoked are:

a) when the ToggleGroup gets its Start() method invoked. This will ensure that a ToggleGroup created in the Editor has a valid state when loaded.
b) when a Toggle is deleted from the ToggleGroup. If the checked toggle is deleted programmatically or via the scene hierarchy, the list must choose a new one.

New Toggles that are added programmatically will fix the state of the ToggleGroup as happened previously.