setup: Fix compile under SDL2.

Use the new API for enumerating screen modes and strip out all code
dealing with pixel depth/bpp configuration.
Track when the window is focused for mouse grab.

We release mouse grab when the window loses focus, but the
SDL_GetAppState() function no longer exists in SDL2. Use window
events to update the screenvisible and window_focused variables.
Fix alpha channel for icons.

Tweak the convert-icon script to include the alpha channel value
as well as RGB. Fix i_video and the setup code to set the icon
including the alpha channel.

While we're at it, replace the current PNG icons with 128x128 high
quality versions so that they look nice on retina display Macs.
textscreen: Change default window title.

Just set this to the empty string.
Remove system specific code for error box.

Use SDL_ShowSimpleMessageBox() which performs the same operation
across many different platforms without needing to rely on
system-specific interfaces.
setup: Fix call to SDL_JoystickName().

SDL_JoystickName() in SDL2 takes a joystick handle rather than an
index. Change calls to this function to match the new API.
Look up key code based on scancode.

Until now we've used the SDL_keycode value to represent physical
keyboard keys. In SDL2 there is a distinction between SDL_Scancode
and SDL_Keycode; the former represents a physical key while the
latter represents a logical key based on software mapping for the
user's keyboard layout.

More work is still needed to rework the keyboard input. In particular,
shifted input doesn't work - the internal event system needs to be
changed to separate key presses from logical text input.