Homepage: Updated Windows download page

No more support for Windows XP and OpenGL 2.0 required.
Fixed|UI|Client: Fatal error when activating splitscreen (viewgrid)

The "ui/background" material no longer exists, as the UI is being
drawn by libappfw and (at least presently) it has no ties to the
material subsystem.

Replaced the obsolete UI background drawer with a black quad.
Documentation: Updated Doxygen configuration

Obsolete config values removed.
qmake|SDL: Upgraded to SDL 2.0, added a dep_sdl2.pri

The old SDL configuration still exists so that older branches can
be built like before. The config variables use a "SDL2_" as prefix
instead of "SDL_", e.g., "SDL2_DIR".

Updating one's config_user.pri is therefore required (SDL_ => SDL2_).
Windows|Installer: Deploy SDL2 and SDL2_mixer libraries
Merge branch 'master' into defs-v2

Conflicts:
	distrib/win32/setup.iss.template
	doomsday/dep_sdl.pri
	doomsday/libdoomsday/libdoomsday.pro