Better option granularity: Toggle between "blacklist" and "whitelist"

DHTML Menu presently allows administrators to disable its functionality for certain menus. However, it cannot yet be enabled only for certain menu; for this use-case, every new menu must be toggled off individually. There is also a slight usability problem involved in having checkboxes implicitly reversed (tick a box to disable).

This new feature proposes to add a switch between "blacklist" (the default) and "whitelist". This makes the setting firstly more flexible, and also makes it clear that when the setting is "blacklist", the checked boxes are the /disabled/ menus.

Changeset is in #473356: DHTML Menu takes another leap and will be isolated into this issue.