Better WAD2 implementation

[ericwa]

• Supports quake and hexen 2 (they both use WAD2, with the palette in gfx/palette.lmp, but the palette colors are different)
• Handles different open documents using different palettes correctly
• If you change the gamedir in the game configuration, the palette isn't reloaded until you close/reopen the document, or open a new document

I didn't add anything to disable this code if the game is GoldSrc; it always tries to load gfx/palette.lmp in the Document constructor, but this could be surrounded by a check for the game engine type.