Ensure up-to-date built-in overlay blacklist.

This changeset makes the following modifications:

 1. The overlay DLL is changed to always consider the built-in blacklist,
    even if a blacklist is present in the registry.
    For Mumble users who have been running with the old blacklist behavior,
    this means that we'll be doing some duplicate checking for blacklist
    entries, but I don't think this matters in practice.

 2. Settings.cpp is changed to not do anything with overlay_blacklist.h.
    Effectively, this means that qslBlacklist now represents the items
    the user has added to the blacklist, and not the combination of both
    an outdated built-in list and the user's own entries.

 3. OverlayConfig.cpp is changed to always show all entries from
    overlay_blacklist.h. Entries from the built-in list are 'disabled',
    so they can't be interacted with.

For more information, see PR #1461
Fix compilation of Overlay.cpp. It was accidentally broken by fd782c3c.