Doomsday Engine

Numerous changes and fixes to get the menu and messages (not the log or chat widgets) working reliably again. Plus a other minor changes:

• Separated message drawing from the menu.
• Separated background fog effect the menu.
• Reordered the fallback responder so that "Press any key" messages work again.
• Redesigned the message system slightly so that it is nolonger necessary to include the response prompt (e.g., "Press Y or N.") in the message string itself. The prompt is now determined in code and a suitable string constructed from the existing text definitions.
• Added text definitions for a few strings that shouldn't have been hard-coded (e.g., "Are you sure you want to suicide?").
• TODO: Fog effect does not currently fade in/out.
• TODO: The "menu slam" needs to be reimplemented.