Load a menu background placeholder in UIMain

UIMain will now load dashbackground.png and checkers.png as a quick
placeholder for the menu's background.
Fix DEBUG #define conflicting with CEGUI

CEGUI would always define DEBUG to 1 unless it was already defined
(Config.h). This would trigger Debug behavior in Release builds.
We change our DEBUG to _DEBUG
Add MainMenuManager, move menu placeholder there

Started implementing MainMenuManager (the onLeveLoad/Unload event
handlers aren't implemented yet, and the menu is still a placeholder)

Move the menu placeholder creation from UIMain to MainMenuManager
Implement UIMain::frameStarted
Improve the menu placeholder
Move & Update media/ folder

The media/ folder now contains everything but the plugins.
Now you just need to compile ponykart, copy the media/ folder in the
build/ folder, and add the compiled plugins in build/media/plugins